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POPULAR SCIENCE REVIEW. 
photographs on wood are no novelty here they are found to he comparatively 
useless, and certainly when used are no improvement. To convert the tones 
of a photograph into lines demands considerable artistic skill and experience, 
such as few engravers bring to their work, and we have seen many engraved 
photographs on wood which were vastly inferior to drawings made from 
photographs by experienced draughtsmen. 
PHYSICS. 
Three New Hydro-electric Piles. — At the meeting of the French Academy 
on the 20th of August, M. Monthier described three new forms of hydro- 
electric piles. The first depends on the employment of sulphuric acid and 
iron. In a cylindrical vessel of iron he places a prism of carbon, and then 
he pours in diluted sulphuric acid. The carbon and the iron form the two 
poles. Two of such batteries are sufficient to ring the ordinary bell 
employed at telegraph stations. This battery is very cheap, and the proto- 
sulphate of iron which results from the reaction may be utilised in battery 
No. 2, which is constructed as follows : — In a cylindrical vessel containing a 
concentrated solution of protosulphate of iron the author places a cylinder of 
zinc and a prism of carbon, forming the two electrodes of the pile. The 
zinc dissolves, hydrogen is disengaged, and hydrated sesquioxide of iron 
precipitated. Two elements of this kind served for an electric bell for 
several months. The third form consists in the employment of putrid 
urine (carbonate of ammonia) and zinc. If, says M. Monthier, you put 
a plate of zinc into a solution of carbonate of ammonia, the metal dissolves, 
hydrogen is disengaged, and forms a precipitate consisting of zincate of 
ammonia and carbonate of zinc. M. Monthier gives a table which shows 
that three batteries with the same number of elements of the same size, 
viz., a pile of “ Marie-Davey ” with sulphate of sub-oxide of mercury, gave 
deflection of the magnetic needle equal to 22 degrees, that of putrid urine 
and zinc gave a deflection of 13°, and that of proto-sulphate of iron and 
zinc gave a deflection of 11°. 
Mode of Correcting Compass Variations. — An extremely simple and 
ingenious mode of ascertaining the deviation, or rather the extent of the 
deviation of the compass, has been devised by an American naval oflicer 
connected with the Naval Academy of Annapolis, Maryland, U.S. He pro- 
poses to take an ordinary compass card and erect upon its centre a fine copper 
wire, from 4 to 6 or 8 inches in height, and perpendicular to its plane. At 
the moment of the sun’s meridian passage, as indicated by the noon obser- 
vation for latitude, note the direction of the shadow cast by the wire on the 
compass card. The angle contained between this direction and that of the 
north and south line of the card will give the variation and local attraction 
combined. Small errors are involved in this method, but the approxima- 
tion is close enough for the purpose for which it is intended. As there 
would, of course, be no difficulty in making this wire a permanent fixture 
to the card, it will be obvious that this arrangement would enable the 
deviation of the compass to be daily tested. — Mining Journal . 
