394 SCIENTIFIC RECORD FOR 1882. 



application to many other problems in thermometry and the measure- 

 ment of pressures and of compressibilities. (Nature, xxv, pp. 00-127.) 



Professor Tait found it necessary to determine the pressure within 

 his Frazer gun to within a certainty of about 1 per cent., correspond- 

 ing to the reading of his thermometers to within 0°.l F. His pressure 

 gauge was founded on the principles adopted by Amagat in his measure- 

 ments of volumes and pressures of gases, and consisted essentially of a 

 small reservoir full of air, the compression of which is measured by the 

 amount of mercury forced into the vessel and retained there by a small 

 self-acting wedge or triangular pyramid of glass. In his later experi- 

 ments the amount of mercury forced into the gauge is shown by the 

 chemical action of the mercury upon a thin film of silver with which the 

 interior of the tube is coated, and the dissolving of which leaves a per- 

 fectly definite record of the distance to which the mercury has advanced. 

 Professor Tait says that he is at present engaged in measuring, by 

 means of a gauge of this kind, the compression of various gases up to 

 pressures of 10 tons to the square inch, or four times those used by Ama- 

 gat. Professor Tait finds that no less than five different causes con- 

 tribute to produce errors in the Challenger thermometers when tested 

 in this apparatus, namely: 1st. The direct effect of external pressure 

 upon the exposed part of the thermometer tubes; 2d. The heating of 

 the water by compression ; 3d. Heating due to friction within the pum^) ; 

 4th. Heating due to the compression of the massive vulcanite slabs em- 

 ployed in the mounting of the thermometers; 5th. The temperature 

 effect produced by ijressure upon the protecting bulb surrounding the 

 true thermometer bulb. This latter is the most difficult of all ; but of 

 these four causes which are active in the experimental apparatus, only 

 one is present in the actual use of the thermometer in deep-sea sound- 

 ing, for in the latter case the heating of the water and the vulcanite by 

 compression and pumping is absent. Therefore, as a final conclusion, 

 Tait asserts that if the Challenger thermometers had had no aneurisms 

 (or enlargements of the bores of the tubes), the amount of corrections 

 to be applied to the minimum index would have been somewhat less 

 than 0°,5 F. for every ton of pressure or for every mile of depth. 

 {Nature, xxv, pp. 90, &c.) 



Michelson describes a very sensitive thermometer, in which the motion 

 of a mirror is effected by hardened caoutchouc. {Nature, xxv, p. 615.) 



As a practical application of the diurnal fluctuations of temperatures, 

 we note the invention of Biji, who has constructed an apparatus by 

 means of which these variations excite thermo-electric currents, which 

 in their turn are continually winding up the driving weight or spring 

 of a clock. (It may be remarked that this is a pretty clear case of per- 

 petual motion, depending for its efficiency principally on the diurnal 

 rotations of the earth.) {Nature, xxvi, p. 384.) 



Eev. F. W. Stow describes a new metal screen to replace the Steven- 

 sou wooden screen, from which it differs only in the following particulars: 



