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increase in the number of sanctuaries, and we trust his appeal will not fall on deaf ears.

THE British Museum of Natural History and the Zoological Society are both setting a good example in their endeavour to arouse in the public a more lively sense than hitherto of the dangers which beset us, and especially this year, from the flies which invade and infect our food and drink. In the central hall of the Natural History Museum Dr. W. G. Ridewood has displayed a small case containing most realistic models of various kinds of food and drink and kitchen refuse, all of which are being partaken of by hosts of flies. Clearly-worded labels and diagrams enable the visitor to assimilate at a glance the nature of these scourges and the best way to combat them. Prof. H. Maxwell-Lefroy, at the Zoological Gardens, has entered much more fully into the matter, having prepared an exhibition of living flies and their larvæ, supplemented by specimens preserved in spirit, with samples of various kinds of kitchen refuse which serve as a nidus for these pests. Wall diagrams and lantern slides arranged as transparencies, showing the different kinds of flies and the essential details of their anatomy, add materially to the usefulness of this part of the exhibition. Having thus demonstrated the magnitude of this menace to the community, he proceeds to point out various preventive measures by means of fly-traps, poisons, and fumigatories. Specimens of each are ranged around the room, and all have the merit of being at once cheap and easily and safely used by the careful housewife.

THE Journal of the Royal Society of Arts for April 23 contains a paper by Mr. Moreton Frewen on the State and the fisherman. After dealing with instances of successful fish culture by the United States Fish Commission, Mr. Frewen proceeds to consider the marine fisheries of British Columbia. These are very prolific. There are extensive feeding and spawning grounds for herring, cod, halibut, and many other food fishes, while salmon are very abundant in the great rivers. During the last year or so fairly large quantities of North Pacific halibut have been exported to this country. Dealing more particularly with the Charlotte Islands, the author contends that there is, in that area, a magnificent opportunity for fisheries development on a large scale. Railway and harbour developments in recent years seem to solve the problem of profitable distribution of the fish reared, caught, or canned. A large part of the paper is devoted to the consideration of the establishment of fisher-colonies in Britisn Columbia, formed from partially disabled ex-soldiers and ex-sailors, and from those more adventurous men who will return to civil life on the conclusion of the war. Everything, he contends, points to the great commercial development of this part of our overseas dominions, provided that the State may foster, by scientific investigation and well-planned emigration proposals, its great natural


THE water relationship between the soil and the plant has been the subject of numerous investigations,

and it has received further attention from Pulling and Livingstone in a publication recently issued from the Carnegie Institution of Washington (No. 204). One of the authors had previously insisted that the power of the soil to deliver water to root surfaces is the prime external condition determining the moisture supply to plants in normal soils. An osmometer was therefore constructed to obtain information on the

phenomena involved, and measurements were taken showing the rate at which water passed from the soil into the cell. For this purpose the large end of the thistle funnel was closed with a collodion membrane obtained by evaporation of a solution of Schering's "celloidin" in a mixture of equal parts of alcohol and ether. This membrane, when properly made, was found to be practically impermeable to dissolved substances, although it readily allowed the passage of water. A cane-sugar solution was introduced, and the instrument was buried in the soil with proper precautions to ensure continuous contact. The rate at which water entered was assumed to indicate the power of the soil to deliver water to the root. It is admitted that the phenomena are complex, but the authors urge that in the present stage more good will result from a direct study of the property as such than from any attempt to analyse it into its component factors. Temperature was found to be of great importance in determining the water-supplying power. A critical point was also found which is said to be approximately the same as that observed by other investigators.

ITALY has taken in hand the study of its own glaciers, and its Società per il Progresso delle Scienze publishes the first number of a Bolletino del Comitato glaciologico italiano (Rome, 1914, pp. 1-114, with illustrations). This, after a prefatory statement, gives a bibliography of Italian glaciology from 1895 to 1913, and some reports on investigations in 1913. Prof. A. Roccati describes some glaciers in the Maritime Alps, the largest of which begins at 9219 ft. and ends at 8662 ft. This, like other Alpine regions where the summits rarely attain to 10,000 ft., should afford good illustrations of upland valleys from which glaciers have not long disappeared. Prof. F. Porro furnishes a preliminary report on the Italian Miage Glacier, which has long been noted for the size of its moraines and its marked advances and retreats. Prof. D. Sangiorgi describes the glaciers eleven of them considerable, from the Disgrazia to the Monte di Zocca. An easily recognised granite occurs in this mountain group, boulders from which are found even to the south of Como. In the Monte Rosa group Dr. Monterin deals with the Lys Glacier and that of the Val d'Ayas, of which the former, like the Gorner Glacier, is still retreating; and the Bolletino ends with studies of two glaciers on the Weisshorn or Corno Bianco (10,893 ft.), a peak on the ridge separating the above-named valleys. Altogether a very promising first number, but we hope that the Comitato will keep in touch with the Zeitschrift für Gletscherkunde, for a multiplication of separate periodicals is apt to increase rather than diminish the difficulties of those interested in the history of glaciation.

THE Società per il Progresso delle Scienze publishes in two small parts a summary (Spedizione Asiatica) of the remarkable work done by Dr. Filippo de Filippi in the Karakoram-Himalayas, but as this is even briefer than the accounts which have been already given in the Geographical Journal, we may await the volume, which will in due course appear, to supplement the one which described the exploring journeys in which he took part under the leadership of the Duke of the Abruzzi. Evidently Dr. de Filippi with his well-equipped expedition has succeeded, during an expedition lasting about sixteen months, in adding greatly to our knowledge of the physiography and petrology of a region in which Mont Blanc would be an inconsiderable peak.

IN interferometers it has been found convenient to introduce into the path of one of the two interfering beams a plate of plane parallel glass of which the angle of inclination to the beam traversing it could be varied. By this means the optical length of the path could be increased by increasing the inclination of the plate to the path traversed. According to a note by Mr. L. H. Adams, of the U.S. Geophysics Laboratory, published in the Journal of the Washington Academy of Sciences for April 19, a complete theory of the action of such a plate has not previously been available, and he supplies it. A number of important deductions may be made from the theory, one of which is the small effect of the index of refraction of the glass on the sensitiveness of the plate as a compensator.

A RECENT paper by Mr. W. Calder on oil-well engineering, read before the Institution of Petroleum Technologists on April 30, is well worthy of the attention of all interested in this highly specialised branch of engineering, and perhaps especially of young men who may be intending to devote themselves to this particular department. In this country, where no practical experience in oil-well technology can be obtained, there is perhaps too great a tendency to regard the subject almost exclusively from the geological viewpoint, and this paper comes opportunely to lay emphasis upon the engineering aspect of the work. It makes it very clear that, although for the location of the borehole the services of the geologist are indispensable, yet all the subsequent work depends for its success upon the engineering abilities of the man in charge. Well-drilling is purely a matter of mechanical engineering, and requires for its successful execution not only a thorough mechanical training, but an amount of resourcefulness and ingenuity that can never be acquired unless by men in whom the engineering instinct is innate. Mr. Calder's paper brings the whole subject well up to date, and describes a 'number of the most modern drilling devices, which, though quite familiar to oil-well engineers, are by no means adequately dealt with in the general literature of the subject; it is therefore an exceedingly useful contribution to oil-well technology.

THE twin sciences of heating and ventilation have received little attention in their scientific aspects. In

a paper read before the Society of Engineers on May 11, Mr. A. H. Barker dealt with a few of the difficulties of these subjects. He pointed out that physiologists are not even agreed upon what was a healthy temperature for human beings to live in. The heating and ventilating engineer aims at producing comfort, but is baffled by the fact that a man is comfortable only when he thinks he is. Air which, judged by chemical analysis, is impure, may feel fresh and sweet, and vice versa. The only legitimate function of the engineer is to produce and control specified movements of air and other effects, while it should be the duty of the physiologist and hygienist to specify what are healthy and comfortable conditions. In connection with heating, the expression "temperature of a room" is generally understood to mean the reading of a thermometer suspended in the room, but this reading does not necessarily indicate the temperature of the surrounding air, or form a trustworthy guide to the feelings of the occupants of the room. The air temperature, the radiant temperature, the quantity of convected heat and the quantity of radiant heat must all be determined, but first the relation between the thermometer reading, the air temperature, and the radiant temperature must be determined. The freshness of air in a building depends on keeping the air temperature relatively low and the radiant temperature high. The chemical composition of the air has, within wide limits, no effect on the human organism, whereas its temperature and humidity are very important. The paper described experiments made at University College, London, and the apparatus used in connection with them, and discussed briefly some problems which it is sought to solve experimentally.

A PAPER on the distribution of heat in the cylinder of a gas engine was read before the Institution of Mechanical Engineers on May 14, by Prof. A. H. Gibson and Mr. W. J. Walker. The paper gives account of a series of tests made at University College, Dundee, in order to determine the jacket loss at different speeds. The engine, built by the National Gas Engine Co., Ltd., has a cylinder diameter of II in., and a stroke of 19 in. The water jacket is divided into two separate parts, one of which surrounds the exhaust-valve, and the other surrounds the breechend and barrel of the cylinder. With the strongest mixture, and at full load, the percentage heat transmitted to the cylinder jacket is 1-10 times as great at 150 revs. per min. as at 250 revs. per min., while with the weakest mixture the ratio becomes 1.23. At 150 revs. per min. the period of contact per cycle, of hot gases and cylinder walls, is 1.66 times as great as at 250 revs. per min., and the rate of transmission of heat is evidently much greater at the highest speed. The indicator diagrams show that the maximum temperature attained in the cylinder is approximately 6 per cent. greater at 150 than at 250 revs. per min., so that the increased rate of transmission is obtained in spite of a lower gas temperature. The reason lies apparently in the fact that the greater turbulence of the working fluid at the higher speeds increases its effective conductivity to an extent which more than counter. balances the other factors.

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In an appendix to No. 4800 of the Astronomische Nachrichten a telephone message to the Computing Bureau is reported from Prof. R. Schorr, stating that the nucleus of this comet has become divided into two parts, their magnitudes being 11 and 13, distance 20", and position angle 291°. This fact was recorded on photographs taken by Dr. H. Thiele with the reflecting telescope of the Bergedorf Observatory.

PHOTOGRAPHIC DETERMINATIONS OF STELLAR PARALLAX. With the hope of making more accurate determinations of stellar parallax by means of photography, Mr. Adriaan van Maanen has been utilising the 60-in. reflector of the Mount Wilson Solar Observa

tory for this purpose. At present he has only completed the discussion of fine stars, and these promising results are briefly described in the Proceedings of the National Academy of Sciences (April 15, vol. i., No. 4). The following table gives the names of the stars, the parallaxes derived, the probable errors, and the number of exposures :—

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REPORT OF THE KODAIKANAL AND MADRAS OBSERVATORIES. The annual report of the director, Mr. J. Evershed, of the Kodaikanal and Madras Observatories, for the year 1914, has just come to hand. Reference is made in the first place to the expedition to Kashmir to test the suitability of the climate for solar research, the result of which was recorded in this column on December 24 last. A long list is given of the instruments belonging to the observatory, which list includes the instruments received from the Takhtasinghji Observatory at Poona. During the year a

positive on negative spectrum comparator, constructed by Messrs. Hilger from designs by the director, was received, and also a diffraction grating ruled by Anderson, with ruled surface 9.7 x 12.8 cm., and total number of lines 75,085. This grating was immediately mounted in the large spectrograph, and "is the most perfect the observatory possesses, and it is now used in all researches where high resolving power is required." The work accomplished by the various instruments is next described. Under the heading of the spectroheliograph it is stated that the measures made from spectroheliograms will in future replace detailed observation at the telescope of the position angles and heights of the prominences. Summaries of sun-spot and prominence observations are given, showing that the solar activity was at a minimum in 1913. Eleven bulletins were published during the year. The report of the Madras Observatory describes the time observations and a general account of weather summary. The appendices contain tables of the seismic records at Kodaikanal and meteorological observations at both Kodaikanal and Madras.

ASTRONOMY AT HAMPSTEAD.—In the year 1899 the Hampstead Scientific Society was promoted for the study and encouragement of a popular interest in science, and at the present time there are three special sections of the society, namely, astronomical, natural history, and photographic. The report for the past year gives one a good account of the activity of the society, but, like many other societies, the disturbing influence of the war has been responsible for a smaller output of work. During the latter part of the year the enforced closing of the observatory rendered any observations impossible, but the attendance of members at the meetings fortunately did not suffer. The report of the astronomical section refers to the observational and theoretical work in connection with the planet Saturn, the new subsection of lunar detail, and the eclipse of the sun in August last. Brief accounts are given of the papers read at the meetings.



WO recent numbers of the Journal of Economic Biology (vol. ix., 3, 4, 1914) contain an interesting article on the biology of sewage disposal, by J. W. Haigh Johnson, who points out that in sewage filters "the varying intensity of pollution between the crude sewage and the purified effluent would provide suitable conditions for the development of a range of organisms." The flora consists mostly of bacteria such as Bacillus coli, Zooglea, and such fungi as Saprolegnia and Sporotrichum. On these feed larvæ of small hairy flies belonging to the genus Psychoda, which often swarm around sewage outfalls. Enormous numbers of the small blue-black springtail Achorutes viaticus form a characteristic and interesting feature on the surface of the liquid.

In the Journal of Agricultural Research, published by the United States Department of Agriculture, valuable biological papers constantly appear. In vol. ii., No. 6, E. O. G. Kelly describes a new sarcophagid parasite of grasshoppers. The female Sarcophaga "strikes " at the grasshoppers (species of Melanoplus and Schistocera) when flying, causing them to drop to the ground. She deposits tiny active maggots beneath the grasshopper's hind-wing. Thence they crawl to the metathorax and enter the victim's body through the soft cuticle at the wingbase.

The same number of the Journal contains an account, by F. Knab and W. W. Yothers, of Toxotrypana curvicauda, the "Papaya fruit-fly," the female of which, by means of an ovipositor as long as her

body, lays eggs deep in the unripe fruit of the Papaya, where the maggots find an abundant and congenial food-supply. This mode of feeding, as is well known, is practised in oranges, lemons, peaches, etc., by larvæ of the notorious "Mediterranean fruit-fly (Ceratitis capitata), which forms the subject of two papers in the Journal (vol. iii., Nos. 4 and 5), by E. A. Back and C. E. Pemberton. This fly is now a serious pest in the Hawaian Islands, though it has not yet been introduced into the United States. Attempts are being made to introduce parasitic and predaceous insects that may keep the fruit-fly in check, and one of the papers just mentioned deals with this aspect of the question. Prof. F. Silvestri made last year a special journey to West Africa in order to study the natural enemies of fruit-flies, the results of which are described in the Boll. Lab. Zool. Scuola Agric. Portici (vol. v., 1914). Here may be found diagnoses with structural figures of a number of the destructive flies and of insects which prey on them, most of the latter being small Hymenoptera.

Our own Bulletin of Entom. Research maintains the high standard of its systematic and economic papers. In vol. v., pt. 3, lately issued, Dr. W. A. Lamborn's account of agricultural pests in Southern Nigeria opens new ground, and is well illustrated with a coloured plate of Lepidoptera and several photographs of injured plants. Stanley Hirst describes mites-mostly Dermanyssus and other Gamasidafound on rats in Egypt; while Rev. Jas. Waterston, turning for a while from Mallophaga, gives an account, with interesting structural figures, of new Chalcidoid fig-insects from Uganda. In the March number (vol. v., No. 4), just received, Mr. Waterston describes a number of parasitic Chalcidoids from tropical Africa and members of the same group from Ceylon. Among several useful papers in this number, S. A. Neave's account of the Tabanidæ of Southern Nyasaland, with descriptions of the early stages and bionomics of the species, is noteworthy. The Review of Applied Entomology is continued monthly, and contains excellent summaries of papers published in all parts of the world, agricultural entomology being contained in Series A, medical and veterinary subjects in Series B. In placing these publications within the reach of all students, the Imperial Bureau of Entomology abundantly justifies its existence. G. H. C.



HE annual report upon the mineral resources of the Philippine Islands for the year 1913 has just been issued at Manila by the Division of Mines of the Government of the Philippine Islands. It gives evidence of fairly steady progress in the development of the mineral resources of these islands under American auspices. The value of the mineral production is estimated at just about 400,000l., an increase of about 14 per cent. above that of 1912, but it must be remembered that these figures are made up, in accordance with the usual practice of the United States, from a number of items that are not generally included in the mineral statistics of other nations, such items as clay products, sand and gravel, and lime accounting for fully one-fourth of the total value. It is noteworthy that no coal was produced in the year under review, whereas the output for the year previous had been 2700 tons; there seems to be no good reason for this complete cessation of coal mining; it is true that one of the mines that had produced coal in 1912 had been drowned out apparently through careless driving into broken ground, which seems not to have been properly tested before the drift in question was put in.

Nearly one-half of the total value of the mineral output is due to the production of gold, of which 42,011 ounces were obtained, both quartz mines and alluvial mines having contributed to the total. The increase of production over 1912, when 27,582 ounces were produced, is relatively important; it is mainly due to dredging operations in the Pasacale district, where five dredges were at work, and where three additional ones were being constructed; it appears that the alluvial deposits are quite satisfactory, but that a shortage in the fuel supply, which appears hitherto to have been wood exclusively, threatens to become a serious problem in the near future.

The output of iron, in the form of castings direct from small native blast-furnaces, amounts to 227 tons, an increase of practically 50 per cent. above that in 1912. These castings are almost exclusively ploughshares; they were produced in ten furnaces, each of which averaged sixty-six days in blast throughout the year; they consumed 555 tons of 60 per cent. ore and 960 tons of charcoal, the iron extracted being thus just about two-thirds of the total iron present. It is interesting to note that this primitive method of ironsmelting is still able to hold its own in the face of imports of iron more than ten times as great as the total native production.


BRISTOL.-The University has been concerned since its foundation with the question of residential accommodation for its students. Early in its history it received an important benefaction for the accommodation of women students in the shape of Clifton Hill House, to which the adjoining Callander House has since been added. During the past year the Imperial Hotel and a large property on Richmond Hill, Clifton, have been purchased for conversion into halls of residence for men and women training students. The University has now taken two houses for the purpose of a temporary residential college for men students. These houses are being renovated and decorated, and will be opened in good time for next term. When the arrangements are complete, the temporary college will start with accommodation for twenty-nine students.

GLASGOW. His Majesty in Council has approved the Ordinance of the University Court, empowering the University to establish a degree of bachelor of science in applied chemistry. In conjunction with the Royal Technical College, which is affiliated to the University, courses in the various branches of chemistry relating to a wide range of arts and industries will be provided. The curriculum extends above four years, and the examinations will be of an honours standard.

More than 140 students in arts, law, and pure science have volunteered for service in the munition factories during the summer vacation. Arrangements have been made by the Appointments Committee with a number of firms on the Clyde, which are prepared to give employment of the kind to students who are not eligible for active service with the forces. When the sessional examinations are completed at the beginning of June, it is expected that at least 100 more students will undertake similar work.

LONDON.-A new edition of the University College "Pro Patria" is in course of preparation, and will be issued shortly. Past and present students, or their relatives and friends on their behalf, are invited to send full particulars of the capacity in which they are serving the country at the present time. In the case of the Army, rank and regiment should be given; in

the case of the Navy, rank and ship. These particulars should be addressed to the Publications Secretary, University College, London (Gower Street, W.C.).

OXFORD.-The Halley Lecture will be delivered on May 20 at 8.45 p.m., in the hall of Queen's College, by Sir Frank W. Dyson, F.R.S., Astronomer Royal. The subject of the lecture, which will be illustrated with lantern-slides, is "The Measurement of the Distances of the Stars."

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THE Year Book Press, 31 Museum Street, London, has been appointed by the Teachers' Registration Council as publishers of the first Official List of Registered Teachers," which will be issued as soon as arrangements have been completed.

IN the issue of Science for May 7 it is announced that Mr. Andrew Carnegie's gifts to the Carnegie Institute and Institute of Technology-both at Pittsburgh-have now reached a total of 5,400,000l., his latest contribution, announced on April 29, being 540,000l. Of this latter amount 240,000l. is for new buildings, and 300,000l. for endowment. From the same source we learn that the campaign to raise 277,000l. for the Stevens Institute of Technology in Hoboken, N.J., has been concluded successfully. The entire indebtedness of the college, amounting to 77,000l., has been cancelled, leaving 200,000l. to be used for the erection of new buildings and for endowment. Gifts amounting to 14,580l., to be devoted to cancer research at the Harvard Medical School, have been announced. Of this sum 10,000l. is provided by the will of the late Mr. Philip C. Lockwood, of Boston. The legislature of Nebraska has granted 30,000l. for the erection of a teaching hospital for the University of Nebraska College of Medicine at Omaha, Nebraska.

THE Board of Education has issued a table of summer courses in England for instruction in many of the subjects of the school curriculum. The courses will be held on various dates during July, August, and September next. Nature-study appears to be one of the most popular subjects, and courses in it are being arranged at Ambleside, Bingley, Brighton, Falmouth, Great Yarmouth, Scarborough, Glastonbury, Newport, and Swanley. Geography, too, seems to be in great demand, probably in view of the growing popularity of practical methods of teaching the subject. Geographical courses are being held at Ambleside, Brighton, Cambridge, Oxford, and Sheffield. Courses in science will be held at Oxford, Wye, and Bangor, and at five centres there will be lectures on the theory of education. The official table states the authorities responsible for the courses, dates, fees, subjects of instruction, addresses for further particulars, and gives useful general remarks. Copies can be obtained through booksellers at a cost of one penny each. The war will reduce the facilities for foreign travel, and teachers who desire to combine further study with their recreation during the long vacation should study this pamphlet.



Royal Society, May 13.-Sir William Crookes, president, in the chair.-Elizabeth A. Fraser and Prof. J. P. Hill: The development of the thymus, epithelial bodies and thyroid in the vulpine phalanger (Trichosurus vulpecula).-Elizabeth A. Fraser: Some observations on the development of the thymus, epithelial bodies, and thyroid in Phascolarctos, Phascolomys, and Perameles.-J. H. Brinkworth Measurement of the specific heat of steam at atmospheric pressure and 104.5° C., with a preface by Prof. H. L. Callendar. The

measurement of the specific heat of steam in the immediate neighbourhood of 100° C. permits extreme steadiness in the conditions of observation, and is important as the starting point for the investigation of the variation of the specific heat with pressure and temperature, but presents special difficulties owing to the possible presence of water in suspension when the superheat is very small. The majority of determinations, such as those of Regnault (1250-225° C.), and Holborn and Henning (110°-270° C.), have been made with highly superheated steam, and throw little light on the value near 100° C. Those of Knoblauch and Jacob, and Knoblauch and H. Mollier, when extrapolated towards saturation, appear to indicate a very rapid increase in specific heat near the saturation point. The theory of the variation of specific heat with pressure is discussed in the preface in relation to some experiments by the Joule-Thomson method, the results of which were published in a previous communication. It is shown that the presence of only half-a-millionth of a gram-molecule of salt per gram of steam is sufficient to raise the apparent specific heat by 10 per cent. at 103° C., and that previous measurements near saturation were probably affected to a slight extent by this source of error. Special precautions were taken in the investigation to secure pure dry steam, and the conditions of experiment were varied widely, especially with regard to external heatloss. By using a silvered jacket of silica maintained at a high vacuum, the loss was reduced to about a tenth of that in previous experiments, and amounted to only two or three parts in 1000 of electric energy supplied. The final result found was-S=2030 joules per equivalent to 0-485 mean calorie under the same congram per 1° C., at 760 mm. and 104.5° C., which is ditions, and is in good agreement with Regnault's result at 175° C., if allowance is made for the small variation deduced from the experiments by the throttling method.-C. F. Jenkin and D. R. Pye: Thermal properties of carbonic acid at low temperatures.-II. This is a continuation of the paper on the same subject published in the Phil. Trans., A, vol. ccxviii., p. 67. It contains a description of (1) a series of measurements of the total heat of CO, gas from which the specific heats are deduced; (2) a few remeasurements of the total heat of liquid CO2; and (3) a series of throttling experiments on CO, gas. By means of the first series the 0 chart, part of which was drawn in the former paper, is extended over the superheated gas area; its accuracy is then checked by means of the throttling experiments. Graphic methods are described for plotting the results of throttling experiments and thereby checking the specific heats of the gas and position of the gas-limit curve. Finally, an I chart is constructed for CO, based on the measurements described in both papers. To assist in the construction of this chart, a series of theorems connecting the total heat I with the other variables p, v, 0, and are worked out, and their use in checking the accuracy of Io charts for any substances is explained. The authors hope that the new Ip for CO, chart, which extends and corrects Mollier's, may be of some technical value in the refrigeration industry.

Physical Society, April 23.-Dr. A. Russell, vicepresident, in the chair.-Prof. W. B. Morton and Miss Mary Darragh: The theories of Voigt and Everett regarding the origin of combination tones. Voigt connects the existence of difference and summation tones with the fact that the stationary points of the compound vibration-curve, when the primary tones have equal energies, can be grouped in a certain way on sine curves, which recur in the periods of these combination tones. As against this view it is urged (1) that the same points can equally well be grouped on a

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