2,000 volts circuits have been among workmen. There is one
point in connection with this matter, on which Mr. de Tunzelmann
is correct, and that is the time-factor. Time, it is now well
established, has a most important bearing upon the result, and if
a man does catch hold of a 2,000 volts service, it is quite correct
to say that he cannot easily let go and is killed by the passage
of the current for a given time. But this is also true, in many
cases, of a 500 volts continuous-current service, and of a 200 volts
alternating-current service. It is hardly correct to say that
the 10,000 volts service would repel a man's hand; but it will
come out to him, and give him a nasty shock, accompanied
by an equally nasty burn, without the necessity of his touching
the conductor. Surely that can hardly be called safer. The
meaning of this is, with 10,000 volts alternating, the maximum
pressure of which is 14,400 volts, the pressure is sufficient to enable
the spark to pass across a short interval between the conductor and
a man's hand. The high pressure breaks down the resistance of
the air-space. So far as he knew, the only possible explanation of
the report referred to by Mr. Mitchell, that high pressures are
safer, is the fact also referred to at Glasgow, that high pressures
are perfectly safe, provided that they are alternating, and at very
high frequencies. He (Mr. Walker) might mention that alter-
nating currents of very high pressure and very high frequency
are being used in London for medical purposes in the treatment.
of cancer and some other diseases. The pressures used cannot be
less than 100,000 volts, and the frequencies probably several
millions per minute, as against the 2,400 per minute, or 40 periods
per second, of modern alternating-current practice.

DISCUSSION OF MR. A. RATEAU'S PAPER ON "A
REGENERATIVE STEAM-ACCUMULATOR, ETC."*

Mr. A. RATEAU wrote that steam-engines, working intermittently, such as winding-engines and rolling-mill engines, even when they are provided with condensers, consume a very large quantity of steam, the consumption being often four times as great as that of an engine working continuously. The principle of the Rateau system is to fit engines working intermittently with a condenser and to cause the steam that enters the latter to do

Traus, Inst, M.E., 1901, vol. xxii., page 613.

work in a turbine, regulating the flow of steam by interposing between the turbine and the intermittent engine, an apparatus which can be called a steam-regenerator.

Taking as an example, the plant nearly completed at the Bruay mines, the steam-regenerator consists of 3 vertical cylindrical vessels of wrought-iron, within which are placed a series of cast-iron plates, the upper portions of which form a bason. These vessels communicate on one side with the cylinders of the winding-engine and on the other side with the turbine. When the winding-engine is at work, the steam enters the regenerator in quantity greater than the mean consumption of the turbine; and the excess is condensed in the cast-iron basins by reason of a slight increase of pressure within the apparatus, a pressure practically equal to that of the atmosphere. The temperature also rises, but this increase is limited to a few degrees on account of the large mass of cast-iron (40 tons) which forms, so to speak, a heat fly-wheel. When, on the other hand, the winding-engine is stopped, the water condensed in the basins is given off again in the state of steam, owing to the heat which has been stored in the cast-iron during the preceding period. The turbine is thus fed continuously by a flow of steam subject to small variations of pressure, presuming that the periods of stoppage of the windingengine are not too prolonged, and, of course, easily brought to an almost uniform pressure, by means of the regulating valve and governor.

The turbine consists of several wheels, in series, mounted upon the same shaft, their diameter being 35 43 inches (0'9 metre). It receives steam from the regenerator at a pressure very slightly below that of the atmosphere, so as not to interfere with the escape of steam from the winding-engine. From the turbine, steam enters the condenser, in which a vacuum of from 24 to 25 inches (62 to 65 centimetres) of mercury is maintained. Under these conditions, the turbine, making 1,600 revolutions per minute, will develop a force of 300 horse-power. This force is utilized to work a double dynamo keyed to the same shaft, the current from which goes to the main switch-board, to be distributed on a three-wire system.

Whenever the winding-engine stops for a somewhat too lengthy period, precautions must be taken that the turbine, which supplies the electric current, should not stop. This possibility

has been prevented by fitting the turbine with an arrangement which automatically passes live steam directly from the boilers. This is so regulated that, for example, if the turbine works at a pressure of 090 atmosphere absolute, steam will be admitted until the pressure attains to 095 atmosphere by means of a special reducing-valve.

The Bruay turbine has been submitted to exact tests in the shops of Messrs. Sautter-Harlé & Cie. (where the machine was built), running at 1,600 revolutions with a vacuum in the condenser of 24 to 25 inches (62 to 65 centimetres) of mercury, and a pressure of 13 pounds per square inch (09 kilogramme per square centimetre) at a consumption of 40 pounds (18 kilogrammes) of steam per hour and per electric horse-power at the terminals of the dynamo. If the winding-engine takes 100 pounds (45 kilogrammes) of steam per effective horsepower-hour (calculated upon the coals raised), the turbine will produce, in the form of electric current, 2 horsepower per effective horsepower utilized, or, say, 250 horsepower, if the useful effect of the windingengine be 100 horsepower. With a pressure of 1 atmosphere, and a vacuum of 26 inches (67 centimetres) of mercury, the consumption of steam in the turbine will fall to 30 pounds (134 kilogrammes) per horsepower-hour. With live steam at 114 pounds per square inch (8 kilogrammes per square centimetre) utilized direct, the steam-consumption would be less than 18 pounds (8 kilogrammes).

The advantage of a series-turbine is to utilize the whole of the fall of pressure down to the pressure of the condenser itself. A turbine, such as that of Bruay, gives an efficiency, which is almost double that of the third cylinder of a triple-expansion. engine.

UNDERGROUND

AND

DISCUSSION OF MR. G. R. THOMPSON'S PAPER ON
"THE CONNECTION
SURFACE-SURVEYS.”*

OF

Mr. S. J. POLLITZER (Sydney, New South Wales) wrote that, in his experience, there was no case in which the magnetic-needle method should, for even the shortest connection, be employed, as the variation of the needle at each plat was different and occasionally by many degrees, and none of the plat-variations can be Trans. Inst. M. E., 1901, vol. xxii., page 519.

VOL. XXIV.-1902-1903.

33

made to agree with the one taken on the surface. In describing the transit-method, Mr. Thompson omitted to give some explanations which were required in order to make the operation complete, although he correctly transferred an azimuthal line from the bottom to the surface: however, the bottom-line had no initial point with its co-ordinates as yet, whereas the one on the surface had it in the vertical axis of the instrument. If he had said that he marked a third point at the bottom, with the telescope truly vertical, the operation as regards horizontal projection would have been complete; and to be quite complete, he might have stated how he ascertained the depth of his bottom-line below the surface. With regard to his minute calculations on the plumb-line method, Mr. Thompson was correct as to the calculations they were, however, not always necessary in practice, if the disturbing elements could be removed, as he (Mr. Pollitzer) showed in his paper.*

Mr. G. R. THOMPSON (Leeds) wrote that it is interesting to have Mr. Pollitzer's statement regarding his experience with the magnetic needle, and his emphatic testimony to its erratic behaviour underground, which shows that great care should be taken to be perfectly sure of the absence of local attraction before trusting a magnetic survey. When, however, local attraction is known to be absent, and the underground sights are very short, a magnetic survey may be the best that is available with any reasonable amount of labour. In the introduction to his (Mr. Thompson's) paper+ he pointed out the necessity of determining a common coordinate point and the depth, as well as the common meridian, but the paper itself was restricted to the consideration of the degree of accuracy attainable in determining the latter. The details for conducting the different operations are admirably described in Mr. Brough's work on Mine-surveying, while a recent paper by Mr. E. H. Liveing‡ gives every detail in the practice of the transit-instrument method for a deep shaft. He, therefore, gave as little detail of the different processes as possible. Regarding the plumb-line method, he would hardly look upon his calculations as minute as they are only such as to determine the equilibriumpoint of a swinging pendulum, and consist in simply taking the Trans. Inst. M.E., 1902, vol. xxv., page 17.

+ Ibid., 1901, vol. xxii., page 519.

Ibid., 1899, vol. xviii., page 65.

average reading. By putting on differently-weighted plummets, he altered the conditions of the experiment and determined whether there was any disturbance or not. Should not this proof always be given before its absence is assumed? He looked forward with interest to the publication of Mr. Pollitzer's paper, to see in what manner he removes the disturbing elements. At the Tamarack mine, recently, it was found that two plumb-lines were about 1 inch farther apart at the bottom than at the top, the depth of the shaft being 4,250 feet. After various experiments, it was concluded that the disturbance was due to air-currents and the shafttop and all openings were closed, when the lines were nearer together, how much nearer the brief note to which he had access did not state; neither could he judge how much the total disturbance of either of the plumb-lines varied from the true vertical. Mr. Hoskold doubts the reliability of plumb-lines for deep shafts; but is it not in such that well conducted plumb-line methods may be expected to compete with telescopic ones? In the case of a dry rectangular shaft, where a possible plumbed base-line may be from 20 to 24 feet long, while the transit base-line could only be 6 to 7 feet long in the same shaft, and where ventilation can be cut off for the time being, he thought that accurate plumbing could be done and results obtained, which for a deep shaft could only be exceeded in accuracy by a very powerful transit-instrument. The accuracy of the result, moreover, would not be left to chance, but freedom from disturbance would be proved: the method is, however, tedious. The power of the telescope must increase for deeper and deeper shafts when the transit-method is adopted. Mr. Hoskold even admits that the telescope on his 5 inches theodolite is not sufficiently powerful for deep shafts, while claiming for his 6 inches instrument that it is. It would be interesting to know in this connection the power of telescope that the makers place on the respective instruments. He intended the error which he fixed for pointing the telescope to be the outside limit, and was prepared to have it reduced somewhat in the discussion; but he was hardly prepared to concede that in practice it would be more imaginary than real. The fact of a very accurate holing having been made at the Severn tunnel does not in any way prove this, for the conditions were those in which great magnification was not required. A shallow tunnel-shaft is by no means the same as a deep mineshaft.