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POPULAR SCIENCE REVIEW. 
a core weighing 92 pounds, and 84 pounds of wire. The latter may he so 
connected as to he equivalent to a wire of double or six times the cross 
section of a single wire. The secondary coil consists of 280 miles of wire 
arranged in 341,850 turns, and forms a cylinder 37*5 inches long ; its in- 
ternal and external diameters are respectively 9*5 and 20 inches. The total 
resistance is 110,200 ohms. It is wound in four sections, the diameter of 
the wire in the two central sections being *0095, and in the outer *0115 
and *0110 inch. The increased section of the extremities is to allow for the 
accumulated charge that portion of the wire has to carry. A condenser of 
the size commonly employed with a 10-inch coil was found to give the best 
results. It consisted of 126 sheets of tin foil, 18 by 8-25 inches, separated 
by two thicknesses of varnished paper ’0055 inch thick. With five quart 
Grove cells a spark of 28 inches was obtained ; 10 cells gave one of 35 
inches, and 30 one of 37*5, and afterwards of 42 inches. These sparks were 
obtained without any difficulty. With the 28-inch spark a block of flint glass 
three inches thick was pierced. In vacuum tubes this coil produces illu- 
mination of great brilliancy and long duration ; with from 20 to 30 cells, 
and a slow mercury break, the striae last long enough to enable their for- 
ward and backward motion to be perceived directly by the eye. The 
appearance of the striae, when viewed in a revolving mirror, was remarkably 
vivid, even when only two or three cells were employed. 
Evolution of Hydrogen at both Electrodes. — According to Dr. Elsasser 
(“Bericht Berl. Chem. Gesellsch,” January 1877), if a magnesium wire be 
made the anode in a decomposition-cell containing very dilute sulphuric acid, 
with a platinum wire as the cathode, hydrogen gas is evolved at both elec- 
trodes, though only half as much is produced at the anode as at the cathode, 
whatever may be the strength of the current employed. A moderately dilute 
solution of magnesium sulphate gives the same result, but in this case magne- 
sium hydrate is deposited at both electrodes. The quantity of the magnesium 
dissolved at the anode was found to correspond exactly to the amount of 
hydrogen set free there, the volume of hydrogen evolved at the cathode 
being the same as that set free in a voltameter in the same circuit. The 
author believes that the positivity of the magnesium is so increased by the 
current that it combines not only with the oxygen set free by the current, 
but also with additional oxygen, thus setting free the hydrogen which was 
combined with the latter. He is, however, unable to explain why the 
amount of hydrogen thus evolved should be exactly half that produced at 
the cathode. — Silliman’s Journal , March 1877. 
New Deep Sea Sounding Apparatus. — M. C. Tardieu has communicated 
to the French Academy of Sciences the description of a new sounding appa- 
ratus invented by him. The apparatus consists of a hollow sphere of india- 
rubber, several centimetres in thickness, communicating with an iron 
reservoir, by means of a narrow tube furnished with a valve. The india- 
rubber sphere being filled with mercury, any increase of pressure forces into 
the iron reservoir a certain quantity of mercury, the return of which is pre- 
vented by the valve. When this apparatus is let down into deep water the 
weight of the mercury in the reservoir will serve as an indication of the 
pressure to which it has been subjected, and consequently of the depth 
attained. — Comptes rendus, 5 February, 1877. 
