Diminished Difference of Thermometers in upper Air. 213 



theory of geological climate, and shows that the approach of our 

 solar system to other systems which that theory implies would 

 seriously and permanently disturb the orbits of the planets. 

 Such a result had been pointed out by Mr. Hopkins in 1852 ; 

 but at the Meeting of the British Association at Manchester in 

 1861, in the course of a discussion on a paper by Sir W. Thom- 

 son, I showed that our system could not approach another 

 system so as to increase terrestrial temperature to the extent of 

 time required by geological phenomena, without the production 

 of a double star group of which our sun would thenceforth be a 

 member. Mr. Croll was probably not acquainted with this cir- 

 cumstance, as my speech appeared (as far as I am aware) only 

 in the local press and in a volume subsequently printed at Man- 

 chester. 



I am, Gentlemen, 



Your faithful Servant, 



Henry Hennessy. 

 Dublin, February 5, 1867- 



XXVIII. On the Reason why the Difference of reading between a 

 Thermometer exposed to direct Sunshine and one Shaded Dimi- 

 nishes as we ascend in the Atmosphere, By James Croll*. 



THE remarkable fact was observed by Mr. Glaisher, that the 

 difference of reading between a black-bulb thermometer 

 exposed to the direct rays of the sun and one shaded diminishes 

 as we ascend in the atmosphere. On viewing the matter under 

 the light of Professor TyndalFs important discovery regarding 

 the influence of aqueous vapour on radiant heat, the fact stated 

 by Mr. Glaisher appears to be in perfect harmony with theory. 

 The following considerations will perhaps make this plain. 



The shaded thermometer marks the temperature of the sur- 

 rounding air ; but the exposed thermometer marks not the tem- 

 perature of the air, but that of the bulb heated by the direct 

 rays of the sun. The temperature of the bulb depends upon 

 two elements : (1) the rate at which it receives heat by direct 

 radiation from the sun above the earth beneath and all sur- 

 rounding objects, and by contact with the air ; (2) the rate at 

 which it loses heat by radiation and by contact with the air. As 

 regards the heat gained and lost by contact with the surrounding 

 air, both thermometers are under the same conditions, or nearly so. 

 We therefore require only to consider the element of radiation. 



We begin by comparing the two thermometers at the earth's 



* Communicated by the Author. 



