192 Prof. Tyndall on the Black-bulb Thermometer. 



"We learn," continues Mr. Wilson, "one more marvellous 

 function of our atmosphere before unsuspected. We knew that 

 it and the aqueous vapour it bears diffused through it save us 

 from loss of heat. We did not know that but for it, or some 

 part of it, we should have no heat to lose. Hence, too, we learn 

 that conclusions as to the temperatures of other planets are still 

 more open to uncertainty, as being in this way also influenced by 

 their atmospheres." 



These are very weighty inferences. They affirm that what is 

 heat in our atmosphere, is not heat in stellar space ; that some- 

 thing is emitted by the sun which warms a body surrounded by 

 an atmosphere, but which is powerless to warm the body if not so 

 surrounded. A planetary atmosphere, in short, has a power of 

 transmuting into heat an agency which, prior to its entering the 

 atmosphere, is not heat, and this power of transmutation, if pos- 

 sessed in a very high degree, might raise the most distant planet 

 to a high temperature. 



I must frankly confess that I do not consider these conclu- 

 sions, however fairly drawn, to be representative of natural facts. 

 I am ignorant of the details of Mr. Glaisher's observation, and 

 therefore not in a position to offer any explanation of it. But 

 the following remarks, which are in substance transferred from a 

 paper presented some time ago to the Royal Society, and ordered 

 to be printed in the Philosophical Transactions, may have some 

 bearing upon the question ; or if not, they may bring to the 

 notice of meteorologists a possible defect in their observations 

 on solar radiation, which, as far as I know, has been hitherto 

 overlooked : — 



1. Solar heat as it reaches us consists partly of visible and 

 partly of invisible rays, a portion of the latter possessing very 

 high calorific power. 



2. The ordinary black-bulb thermometer absorbs the visible 

 rays of the sun, but the black glass of the bulb may be highly 

 transparent to the invisible radiation. 



3. The invisible calorific rays, especially, augment in power as 

 we ascend in the atmosphere; for it is those rays which suffer 

 most diminution of intensity during their passage through the 

 aqueous vapour of the air. 



4. Hence the black-bulb thermometer must, with reference to 

 the total radiation falling upon it, become more and more trans- 

 parent as we ascend in the atmosphere. It is not an exaggerated 

 estimate that at the limits of our atmosphere 50 per cent, of the 

 solar heat might cross the glass of the bulb, be reflected by the 

 mercury within it, and contribute in no degree to the heating of 

 the thermometer. 



If these remarks be correct, or so far as they are correct, 



