Prof. Tyndall on the Black-bulb Thermometer. 191 



a small deviation is of very small importance, but of much greater 

 in the case of solids and liquids. It is therefore of urgent ne- 

 cessity to investigate as accurately as possible whether such an 

 ideal state occurs in the case of gases — not because the condition 

 of these gases is of such importance in itself, but because the 

 entire theory of heat, so far as it is connected with the ordi- 

 nary definition of Carnot's function of temperature, stands or 

 falls with it. 



The occurrence of this ideal condition is the more probable, 

 the more the various deviations of actual gases from the ideal 

 condition taken together converge towards nil. As regards 

 Mariotte and Gay-Lussac's law, this follows from Regnault's 

 investigation, and it is confirmed by the testing of the law of 

 volumes ; the values of the internal work given in § VII. show, 

 too, that this decreases with higher temperature and lower pres- 

 sure; this also follows from Thomson and Joule's experiments, 

 which in this respect have great value. So far as previous 

 observations permit, I believe I may conclude, from what has 

 here been communicated, that these deviations decrease together, 

 by which the validity of the assumption of an ideal gas acquires 

 a confirmation. 



Ziitphen, February 1864. 



XXVII. On the Black-bulb Thermometer. 

 By Professor Tyndall, F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the February Number of the Philosophical Magazine, Mr. 

 Wilson of Rugby has published an interesting letter, in 

 which he describes an observation made by Mr. Glaisher, to the 

 effect that the difference of reading between a black- bulb ther- 

 mometer exposed to direct sunshine, and a second thermometer 

 shaded from the sun's direct action, diminishes as we ascend in 

 the atmosphere. This observation points to the following con- 

 clusions which have been drawn by Mr. Wilson. 



1. " If the experiment could be made, the shaded and exposed 

 thermometers outside the earths atmosphere would show the 

 same very low reading/' 



2. " All those views of the state of the surface of the moon 

 which represent it as alternately exposed to fiery heat and in- 

 tense cold will disappear. The temperature on the dark and 

 bright halves of the half moon is the same if there is no atmo- 

 sphere." 



