144 On the "Barometer. 



contrary to reason, that the barometer should be affected 

 by it; since, in proportion as the atmosphere is rarefied, 

 from this cause ; in the same proportion, from the same 

 cause, likewise, is the atmosphere acn/wM/a^e^ there ; and 

 hence the gravity ^ or pressure of the atmosphere upon the 

 surface of the earth, remains the same in each instance. 



I might add, that if an alteration in the barometer 

 actually took place from such a cause, it could scarcely 

 have escaped my notice. 



P.S. I have noticed with considerable sensations of sur- 

 prise and satisfaction, the progress of ProfessorLeslie, in his 

 experiments on the product >07i of' cold by means of Ike air- 

 pump, in which it is stated that that gentleman has already 

 reached a cold of 70" below 0. At the time I was pur- 

 suing my experiments on artificial cold by frigoritic mix- 

 tures, the means of producing artificial cold by means of 

 the air-pump naturally presented itself, and to which I was 

 more particularly stimulated by an experiment related by 

 'Mx.Nairne, in which it is said, by the evaporation of ether, 

 under the air-pump, a cold of 4S^ helow was produced, 

 the temperature of the air in the room being 53°. Pkilo' 

 sophical Transactions 1777- 



I made several attempts at producing an extraordinary 

 degree of cold by this means, but without success; owing 

 chiefly, as I presumed, to the imperfection of the best air- 

 pump I was able to procure. Conceiving that ihc Torricellian 

 vaaium, as it is called, might be the best method of effect- 

 ing the most sudden and complete evaporation of ether, 

 upon which the success of the experiment depends, I had 

 projected in my mind to put this method to the test; but 

 the difficulty of obtaining a fit apparatus for the purpose 

 prevented me. 



Perceiving that Professor Leslie's experiments, in which 

 be has succeeded to a most extraordinary degree, have been 

 conducted upon the principle alone of rarefying the air, 

 without any auxiliary means excepting his ingenious me- 

 thod of absorbing the vapour, the existence of which ne- 

 cessarily diminishes the success of the experi;iient ; I have 

 been induced to take this opportunity of mentioning these 

 circumstances, fully trusting that, should these observations 

 furnish no hint to the learned and ingenious Professor, they 

 will not be deemed an improper interference*. 



• By means of evaporation in the open air, from purified sulphuric clhfr, 

 (as mentioned in my 'J'lcatise on the Pruduclion of CuUl, pages 80, 81,) I sunk 

 the thermometer from 71", the temperature of the air at the time, to 12* > 

 and by water used in a similar manner, to i^"- 



XXVIII. Ko. 



