42 Prof. Tyndall on the Relation of 



sunrise on the former day was 38°, while on the latter it was 41°. 

 Indeed between April and September a range of 40° in clear 

 weather was quite common — or more than double the amount 

 which it is in London at the corresponding season of the year. 



A freedom of escape similar to that from bodies at great ele- 

 vations would occur at any other level, were the vapour removed 

 from the air above it. Hence the withdrawal of the sun from 

 any region over which the atmosphere is dry must be followed 

 by quick refrigeration. This is simply an a priori conclusion 

 from the facts established by experiment ; but, I believe, all the 

 experience of meteorology confirms it. The winters in Tibet are 

 almost unendurable from this cause. The isothermal s dip deeply 

 from the north into Central Asia during the winter, the earth's 

 heat being wasted without impediment in space, and no sun 

 existing sufficiently powerful to make good the loss. I believe 

 the fact is well established that the desert of Sahara, which 

 during the day is burning hot, is often extremely cold at night. 

 This effect has been hitherto referred in a general way to the 

 " purity of the air ; " but purity, as judged by the eye, is a very 

 imperfect test of radiation, for the existence of large quantities 

 of vapour is consistent with a transparent atmosphere. The 

 purity really consists in the absence of aqueous vapour from 

 those so-called rainless districts, which, when the sun is with- 

 drawn, enables the hot surface of the earth to run speedily down 

 to a freezing temperature. 



On the most serene days the atmosphere maybe charged with 

 vapour ; in the Alps, for example, it often happens that skies 

 of extraordinary clearness are the harbingers of rain. On such 

 days, no matter how pure the air may seem to the eye, terrestrial 

 radiation is arrested. And here we have the simple explanation 

 of an interesting fact noticed by Sir John Leslie, which has re- 

 mained without explanation up to the present time. This emi- 

 nent experimenter devised a modification of his differential thermo- 

 meter, which he called an JEthrioscope. The instrument con- 

 sisted of two bulbs united by a vertical tube, of a bore small 

 enough to retain a little liquid index by its own adhesion. The 

 lower bulb was protected by a metallic coating ; the upper or 

 sentient bulb was blackened, and was placed in the concavity of 

 a polished metal cup, which protected it completely from terres- 

 trial radiation. " This instrument," says its inventor, " will at 

 all times during the day and night indicate an impression of cold 



shot downwards from the higher regions But the cause of 



its variations does not always appear so obvious. Under a fine 

 blue sky the JEthrioscope will sometimes indicate a cold of 

 50 millesimal degrees ; yet on other days, when the air is equally 

 bright, the effect is hardly 30°. " It is, I think, certain that these 



