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POPULAR SCIENCE REVIEW. 
mm. long by 8 mm. broad, and 06 mm. thick. In these couples galena is 
the electro-negative element; iron, the electro-positive. The form of the 
bars is such, that by placing them side by side they form a ring of twelve 
couples, of which the interior is formed by the extremities which are to be 
heated. They are united in tension ' by means of tin solder. They are 
isolated from one another by thin mica plates. By placing five of these 
rings in a vertical column a battery of sixty couples is formed. These rings 
are isolated and separated by washers of asbestos. The whole is firmly held 
between two iron rings by means of three bolts. The pile thus forms a 
hollow cylinder, the interior of which must be heated. The cooling of the 
junctions, whose temperature should be lower, is caused by radiation into 
the air. The interior cylinder measures 50 mm. in diameter and about the 
same in height. The heated surface is about 78 square centimetres. The 
apparatus is heated by a gas-burner, consisting of a steel cylinder, 56 mm. 
in diameter, closed above, opened below, and pierced with small orifices. 
Tliis is placed in the centre of the pile. A tube pierced with holes surrounds 
this cylinder, and distributes the gas uniformly around it. The gas rises, 
and arriving at the orifices in the burners, meets the air which is escaping 
from it because of the draught of the tube of steel that surrounds the appa- 
ratus. Each orifice in the burner thus forms a blow-pipe, the jet of which 
strikes the opposite side. Forty couples have an electro-motive force equal 
to that of a Bunsen element. 
A convenient Electric Lamp which deserves the careful attention of lec- 
turers, whether they be exhibitors of spectra or of micro-photographs, is 
that of Mr. Ladd. It is certainly one of the neatest and most regular- working 
lamps we have ever seen. It did its work admirably at the last meeting of 
the Chemical Society, when Mr. Perkin showed the spectrum of Alizarine. 
A Prize for the best System of Ventilation. — The Royal Danish Society 
of Science, among other prizes, has offered one amounting to 34/. for the 
best essay containing an investigation of the movement of the air in a system 
of ventilation. The essay may be written in English, French, German, 
Danish, or Swedish, and must be handed in before October 1870. 
The Vibrations of Tense Strings. — At the meeting of the Vienna Academy 
on January 22, Professor E. Mach sent in a research on the vibrations of 
stretched cords, carried out in the physical laboratory of the University of 
Prague by Herr Clemens Neumann. The author has observed, by very 
different and partly very singular methods, both the movement of particular 
points in the strings and also the movements of the entire strings. 
The Temperature of Lakes. — At the same meeting of the Vienna Academy 
as that above mentioned, Professor F. Simony presented a comparative view 
of the temperature relations of the Hallstatter Lake, Gmunder Lake, and 
the two Langbath lakes, in which he had taken the temperatures at all 
depths at the same periods of the years 1868 and 1869, with the view to 
make out the extent of the influence of the different climatic characters of 
these two years on the temperature of lakes. 
