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one hand, and with carbon and hydrogen on the other, as in the explosives, e.g. nitroglycerine.

Living substance has apparently all the above mentioned sources of instability, and perhaps not the least important is that it has for its pivot nitrogen, the element which above all others is remarkable for the lability of its compounds. I have elsewhere indicated the probability that the active molecule of living substance consists of an enormous complex of proteids, carbohydrates, &c., linked together by means of the nitrogen atoms, and that the oxygen store is more or less combined with the nitrogen. At the death of the molecule its constituent groups (proteids, &c.) are released, and the store of oxygen passes from the nitrogen into other and more stable forms of combination. F. J. ALLEN.

Cambridge.

Chalk Masses in the Cliffs near Cromer. Ar the present time the cliffs near Cromer exhibit some interesting chalk masses in the Glacial drifts. Between East and West Runton Gaps are several of great size and remarkable in position. One, a very long slab-like mass, is bent from being nearly horizontal until it is almost vertical, and thus comes to within a short distance of the top of the cliff. The masses near Trimingham will now repay a close study, for they have changed greatly during the last five years. Both my friend, the Rev. E. Hill, and I have made notes and rough sketches, with the intention of sending to the Geological Magazine a short account of what can now be seen; but we earnestly hope that some geologists who are adepts at photography will visit both localities at the earliest possible opportunity, in order to secure a permanent and accurate record of these exceptionally interesting sections. T. G. BONNEY.

The Rigidity of the Earth's Interior. THE letter of Dr. T. J. J. See (NATURE, April 13, p. 559) deals with a subject of profound interest to students of the larger problems connected with physical geology. But it appears that, in Dr. See's treatment of the subject, he has overlooked an important point, which I dealt with in a paper read before Section C of the British Association at Birmingham in 1886. Therein I directed attention to the fact that " gravitation" is only a special instance of the law of universal attraction, and as a corollary to this, at any considerable depth within the sphere of the earth, an appreciable factor of what I may call negative gravity must be allowed for, owing to the counterattraction of the mass of matter situated nearer the surface of the sphere; so that a body placed at the centre of gravity of the earth, whatever its mass or density, would have no weight at all.

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I am glad to see that the consideration of "critical temperatures of quasi-solids (the importance of which was emphasised in my little work on metamorphism some fifteen years ago) is receiving serious attention, and I may also point out that the idea of a potentially liquid (or even gaseous) condition of a mass at depths in a practically rigid state is not new; it was treated in a masterly way by Prof. Albert Heim, of Zürich, some twenty years ago, in his magnificent work "Ueber den Mechanismus der Gebirgsbildung.' "Ueberlastet " is the word used by Heim to express such conditions, where the pressure is so far "hydrostatic as to consist of compression acting equally (for the time being) in all directions. Any disturbance in a given portion of the lithosphere of the equilibrium thus existing must result in shearing movement if the disturbance be small, and in flow in a given direction if the relief in that direction from pressure is great and rapid enough. In the former case we should get metataxic change," in the latter schistosity; for I still challenge the statement, made recently by a high authority, that "it is only a question of degree between the cleavage of a slate and the foliation of a crystalline schist or gneiss."

Questions relating to tidal action in the rotating lithosphere, and even Lord Kelvin's oft-repeated objection on

1 Report Brit. Assoc., 1896, p. 983; and Proc. Birmingham Nat. Hist. and Philos. Soc., 1899.

that ground to the impossibility of any considerable portion of the lithosphere being fluid, because the earth does not undergo the deformation which the physicist would expect owing to the tidal action which should be set up within it, might possibly be seen in a fresh light on taking into account the remarkable facts demonstrated by Prof. John Perry in his lecture on spinning tops, which he gave to an audience of working men on the occasion of the meeting of the British Association at Leeds in 1890. As a "working man in a real sense of the word, I considered myself privileged to attend that lecture, and was rewarded by finding in my own mind a great difficulty cleared up by Prof. Perry's masterly demonstrations of the practically rigid condition of non-rigid bodies, if only made to rotate with sufficient rapidity, as the equatorial regions of the earth do-something like 1000 miles an hour. Bishop's Stortford, April 17. A. IRVING.

Rival Parents.

A CURIOUS example of the rival claims of a pair of thrushes and a pair of blackbirds for the parentage of a young blackbird is being observed in my garden.

A pair of blackbirds built a nest in a small thick laurel, and in another shrub, some 4 feet off, a pair of thrushes also built a nest. The young in both nests were hatched out at the same time, and were successfully reared until they were some eight or nine days old, when a cat attacked the nests (Monday, April 17), killing all the young thrushes and all the blackbirds except one, which was found hidden under the shrubs. It was continually visited after the tragedy by both the old thrushes and old blackbirds, and two or three hours later was removed in some way not observed to a shrubbery twenty or thirty yards away. There for the last five days it has been fed and looked after by both pairs of birds, who mob with exceptional vigour any intruding cat or dog. The young bird seems to have thriven mightily under the attentions of its true and foster parents, who appear in no way to be jealous of one another. KENNEDY J. P. ORTON. University College of North Wales, Bangor, April 21.

The Measurement of Mass.

IN the notice of my little book, "Radium Explained," on April 6, twenty-nine lines are devoted to showing that I have reached a wrong conclusion through not knowing that mass is measured by inertia, and I am corrected in these words:" how is the quantity of matter to be ascertained? The choice practically lies between defining mass by inertia at a given speed or by gravity. . . . As, however, gravity depends on local circumstances, while inertia (at given velocity) does not, the latter property is preferred for the definition of mass, as being more fundamental." So far from rejecting this principle, I state it, in almost the same terms, on p. 84 of my book :-"Mass, or quantity of matter, is usually ascertained by weighing. But weight is merely the force with which the earth attracts, and this varies with our position on its surface. To get an absolute test of mass, which would be independent of position, we may measure the force required to move or stop a body at a certain speed." And nowhere in the book have I supported any argument by the repudiation of the principle here clearly stated. This is a question of fact; the other objection taken is equally illfounded, but, being on a controversial point, it cannot be dealt with so briefly. W. HAMPSON.

West Ealing, May 1.

Properties of Rotating Bodies. PROF. W. H. PICKERING, in NATURE of April 27 (p. 608), refers to the property which a rotating body possesses of assimilating, in certain circumstances, its axis of rotation to a secondary axis of rotation or revolution impressed upon it, and he mentions the fact that this property is rarely described.

It was fully discussed in an elementary lecture given by Prof. Perry at the Royal Institution about fifteen years ago, and afterwards published in the Romance of Science Series under the title "Spinning Tops." E. W. ROWNTREE.

20 Queen Square, W.C., May 1.

RECENT

IN

SPECTROHELIOGRAPH RESULTS.

Na previous number of this Journal (vol. lxix. p. 609, 1904), under the heading of "A New Epoch in Solar Physics," I gave an account of the magnificent work that Prof. Hale had recently been accomplishing at the Yerkes Observatory with his latest form of spectroheliograph, the instrument being worked in conjunction with the great 42-inch Yerkes refractor, which forms an image of the sun seven inches in diameter.

In the present article it is proposed to give a brief description of another instrument based on the same principle, an account of which was published by M. Janssen, and to indicate some of the results which have been obtained with it. This instrument has been at work at the Solar Physics Observatory during the past year, and in a recent communication to the

which the solar image is moved across the primary slit by means of the declination motor which moves at the same time and rate the photographic plate; or the primary slit, and with it the whole spectroheliograph, may be moved across the image formed at the focus of the equatorial. The first method is that adopted at the Yerkes Observatory, and the second that at' Potsdam.

There is a further method in which a stationary solar image is formed by means of a siderostat and lens, and the spectroheliograph is mounted horizontally and moved in an east and west direction across this fixed image. Such a mode of procedure is that employed at South Kensington.

The advantage of the last mentioned arrangement is that there is no limit to the size or weight of the spectroheliograph; the uniform motion required can be easily and efficiently secured, and lastly, this

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FIG. 1.-The spectroheliograph, showing the general arrangement of the two slits, the collimating and camera tubes, the moving (upper) and fixed

Royal Astronomical Society I gave

(lower) triangular frameworks.

a more full

account of it, to which reference can be made for more detailed information than is here given.

it is not necessary in this place to refer at any length to the principle which underlies the construction of a spectroheliograph, since this was referred to in the article above mentioned. It will suffice here to say, therefore, that the pictures produced by this new method of solar research give us photographs of the sun in monochromatic light, or in rays of any particular wave-length that is desired. Thus if we require to study the distribution of hydrogen on or around the solar disc we employ a line in the spectrum of hydrogen, if calcium a calcium line, or iron an iron line.

There are, however, several methods of using the spectroheliograph. This instrument may either be employed in conjunction with a large equatorial, in

motion does not in any way affect the steadiness of the solar image under examination.

The South Kensington instrument was erected in the year 1903, but it was not until last year that satisfactory photographs were secured and routine work begun. This success was due to the use of a larger lens (12-inch) for throwing the solar image on the primary slit, the previous lens of 6 inches aperture not giving a sufficiently bright image.

In this curtailed description of the instrument reference of any length need only be made to the spectroheliograph proper. There is nothing particularly novel about the siderostat, except, perhaps, its more than usual size, the large mirror of 18 inches diameter, the two small motors for operating the slow motions in right ascension and declination, and a modified form of Russell control for regulating the speed of the driving clock. This instrument is

placed in a separate house the upper portion of which can be rolled back towards the north. Some distance due south of this, in another building, is the 12-inch Taylor photo-visual lens mounted on a concrete pillar, and still further south, and in the same building, is the spectroheliograph, also mounted on concrete pillars.

Wtih this arrangement the solar beam is thrown by the siderostat mirror continuously due south and in a horizontal direction; this beam then falls on the 12-inch lens, and the solar image in the focus of this lens is thrown on the primary slit plate of the spectroheliograph.

In order to analyse the solar image by allowing each portion of it to fall successively on the primary slit, the latter, and consequently the whole of the spectroheliograph, has to be moved horizontally in an east and west direction, a distance a little more than the diameter of the solar image (in this case 24 inches). Further, this motion has to be extremely uniform.

The method adopted to accomplish both of these requirements is as follows:-A triangular iron framework (Fig. 1) is supported on three levelling screws on three concrete pillars. A second framework of the same size and material is placed on the first, but separated by steel balls free to roll between small steel plates fixed to each framework near the corners.

The longer side of this isosceles triangle is placed in a north and south direction. The direction of motion of the upper framework is restricted to an east and west line by means of a guide bar fixed to the lower framework; two small levers with rollers attached to the upper framework are pressed against this guide bar by means of small weights, thus ensuring the correct direction.

The actual motion of the upper framework is obtained by weights attached to one end of a steel strap the other end of which, after passing over a pulley mounted on an arm on the lower framework, is fixed to the western corner of the upper framework. This weight always tends to pull the upper framework towards the west, that is towards the right in Fig. 1.

The motion is controlled by a plunger projecting downwards from the upper framework operating a piston in a cylinder full of oil attached to the lower framework. The outlet valve can be so adjusted that any desired rate of motion can be obtained.

Owing to changes of temperature of the oil, different rates of movement can be obtained for any one reading of the micrometer head regulating the outlet valve. It is necessary, therefore, when making an exposure for a "disc" or "limb" picture to take the temperature of the oil into account. This is accomplished by emploving a table, made from previous "runs,' in which the valve setting can be directly read off from the temperature reading and the required length of exposure.

It is on the upper framework that the optical parts of the spectroheliograph are placed. These consist of a double tube carrying the two slits (Fig. 2) at the northern or siderostat end and the two lenses (4-inch) of equal focal length at the southern end. The dispersion is produced by a single prism of 60°, and a reflector is inserted in the system in order to make the total deviation of the beam 180°. Thus the part of the solar image which passes through the primary slit falls on the collimating lens, is reflected by the 6-inch mirror on to the prism, traverses the latter, and finally, after passing through the camera lens, is brought to a focus in the plane of the secondary slit in the form of a spectrum. By isolating any particular line in this spectrum by means of the secondary

slit (Fig. 2) the solar image can be analysed in this wave-length.

For photographing the whole disc of the sun or its immediate surroundings with one exposure the lengths of the slits must be greater than the diameter of the solar image (2 inches); in the present case they are 3 inches long. Further, owing to the fact that the lines in the spectrum are curved, the secondary slit jaws are curved to the same radius; this necessitates very accurate adjustment of the secondary slit on the line, and means are provided to facilitate such requirements.

In order to obtain a photographic record of the sun in monochromatic light, a fixed photographic plate is held by means of a wooden support as close to the secondary slit as possible (Fig. 2). In this way, as the primary slit moves over the stationary solar image, so the secondary slit traverses with equal speed the stationary photographic plate.

Up till now the secondary slit has usually been

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adjusted on the "K" line of calcium by eye estimation aided by a small watchmaker's lens, a check being made by taking a photograph of the spectrum, if possible with a sun-spot region, on the primary slit. On bright days this setting can be made with little difficulty, but during the late autumn, with a low sun, the "K" region of the spectrum is not easy to see, and the setting is in consequence very uncertain. A new method just brought into operation entirely eliminates this difficulty, for at a constant distance on the red side of the "K" line a small glass plate has been set with a cross engraved on its surface which can be adjusted on a known line in the more visible region of the spectrum. By bisecting a particular line with the cross the "K line is adjusted on the slit jaw simultaneously.

The photographs taken during the past year have been of two kinds, the first to investigate the distribution and area of the calcium clouds, or flocculi as Prof. Hale has termed them, on the sun's disc, and

the second the distribution and forms of prominences round the limb. To obtain the latter, a metal disc just a little smaller than the solar image is placed close up to the primary slit plate (Fig. 2), and retained there by a metal wire fixed to a firm base; this disc is so adjusted that it is concentric with the solar image. While in use it becomes extremely hot, and it is therefore necessary that it be made of metal and riveted to the wire which supports it. These limb pictures, an example of which is given in Fig. 3, are

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FIG. 3-Limb and disc of sun in "K" light, July 19 Limb exposed ron 11h. 36 n. to 11h. 52m. (interval 16 n.); Disc exposed from 11h. 53 n. 30s. to 11h. 53m. 48s. (interval 18..). Enlarged nearly 2 times.

of a composite nature in that after the exposure of the limb has been made the metal disc is removed from the primary slit, and a "disc" exposure is made on the same plate. It has been found by experience that a "limb" exposure requires about sixty times the time that is necessary for a "disc" exposure. Under very favourable conditions fifteen seconds is necessary for the latter and fifteen minutes for the former.

the prominent flocculi, many of which clearly indicate the mode of structure. Fig. 3 gives an idea of their appearance in the photographs. It will be seen that there are frequently long streaky bright portions springing apparently from a central nucleus and having subsidiary ramifications. A three-legged formation is a very common type of structure in many of the photographs.

These flocculi, in the first instance, exist alone, but

in some of them spots appear at a later stage. No spot has been photographed unaccompanied by a flocculus; in fact, the duration of a spot is only a brief interval in the life-history of a flocculus.

Another interesting subject of inquiry is the position of a spot in relation to the flocculus. Spots more generally make their appearance near the head of, or, in other words, precede the apparently trailing masses of the calcium clouds with respect to the solar rotation, which is from east to west. Some examples of these are given in Fig. 4. When there are two fairly large spots in one flocculus, the larger one nearly always precedes the smaller one.

The composite pictures (Fig. 3) showing the

1904

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April 27

April 27

July 14

August 2

FIG. 5.-Showing changes in prominences after an interval of one hour. (Lower picture taken last.)

scribed. In Fig. 5 we have two photographs (only the portions of the limb indicating the particular region of the sun in question are shown) which were taken on July 14, 1904, at 11h. 8m. a.m. and 12h. 8m. p.m. respectively. It will be noticed that during this interval of about one hour a startling change has occurred to the largest prominence; not only has its height been considerably increased, but its form has entirely changed. The material radiating the calcium light seems to have been ejected from the chromosphere and then to have apparently met a strong current moving polewards (that is, from left to right in the figure) which has thrown this material in that particular direction. The change of height from about 50,000 miles to 60,000 miles in this interval corresponds to a velocity of nearly three miles a second.

Not less interesting is the apparent disappearance of the second large prominence in the figure situated on the left.

Another example of a change of form of an enor

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August 29

FIG. 4.-Typical cases of spots situated in the front portions of flocculi.

"limb" and "disc" have also brought to light many interesting points which call for further inquiry. In the first place prominences both near the solar poles and equator give strong images in calcium light. Secondly, prominences, which occur nearer the solar poles than the flocculi, do not appear to disturb the regular mottling on the disc in these high latitudes.

Again, an intense flocculus, when on the limb, is not always accompanied by a large prominence. These two last mentioned facts seem to indicate that flocculi and prominences are not always interdependent phenomena.

On continuous fine days, when several photographs

FIG. 6. Two views of a large prominence taken with a four hours' interval between them. (Lower picture taken last.)

mous prominence photographed on July 19 at 11h. 45m. a.m. and 3h. 59m. p.m. respectively is that shown in Fig. 6. This prominence was situated in the south-east quadrant. The approximate dimensions

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