166 Mr. T. Hopkins on the Causes of the 



4° C; with steel when it amounted to 2° or 3°; and lastly, with 

 lead, when 1° of difference existed, it was not accurate. 



2. The reason for this want of accordance between the ob- 

 servation and the mathematical law is, that in establishing the 

 latter the outward and inward power of conduction of bodies 

 was considered as independent of the temperature. If these 

 be considered as functions of the temperature, a proximate 

 formula for the distribution of heat in the rod may be esta- 

 blished, which would very nearly agree with my observations. 



3. The conducting powers of bodies established by former 

 philosophers with the aid of Biot's law are consequently in- 

 correct, and can only be considered as partial approximations 

 to the truth. 



4. The constant coefficient for the conducting power with a 

 difference of temperature equal 0, is therefore only to be de- 

 termined in this manner ; either by using Biot's law and ob- 

 serving the distribution of heat in the rod with very small 

 differences of temperature, or more correctly, by ascertaining 

 its value, as was done in the author's experiments by means of 

 Poisson's formula. 



5. That the method employed in the observations is accu- 

 rate, and that the thermo-electrical battery will become in the 

 hands of natural philosophers a more correct means of ascer- 

 taining the temperature of the surfaces of bodies than any 

 other, and may be used in cases where the common thermo- 

 meters cannot be employed. 



XXXI. On the Causes of the Semi-diurnal Fluctuations of the 

 Barometer. By Thomas Hopkins, Esq.* 



THAT the non-condensable gases and the aqueous vapour 

 of the atmosphere, when it is at rest, press on the mer- 

 cury of the barometer independently of each other, and con- 

 stitute the general atmospheric pressure, is evident from their 

 known laws of diffusion and independent existence while dif- 

 fused through each other. 



But that the facts and reasonings, commonly adduced, rest- 

 ing on those circumstances, together with the daily alterations 

 of thermometic temperature, account for the two risings and 

 the two fallings of the barometer, as is contended by some 

 parties, cannot be admitted. 



In certain parts, such as Canada, of which an account has 

 been recently given by Colonel Sabine, the influence of the 

 causes named may be sufficient to account for a considerable 



* Read to the Literary and Philosophical Society of Manchester, Dec. 

 30, 1845, and communicated by the Author. 



