Welling ton Philosophical Society. 469 



direct rays of the sun. The calorific rays of the sun pass through air devoid 

 of aqueous vapour with no appreciable loss; but if water in the form of 

 invisible vapour be present, the air is not perfectly transparent to those rays, 

 and offers, I believe, a slight obstruction to their passage. It is almost opaque 

 to radiant heat from the surface of the ground. Transparency to heat and 

 light is witnessed in the passage of the sun's rays through the glass windows 

 of our dwellings. The heat in a close room into which the sun shines may 

 be overpowering, while the glass, through which the whole of the heat 



an- 



has passed, remains cold. The greater the proportion of aqueous vapour 

 the more solar heat is absorbed in its transit through the atmosphere. Now, 

 the quantity of vapour in the air depends mainly on temperature. In the 

 colder regions of the south, although the air may be saturated with vapour, 

 the relative proportion of vapour to air is much less than in tropical climates ; 

 and thus it happens that in Southland less of the sun's heat is lost in its 

 passage to the earth. For instance, — the quantity of vapour 

 temperature of 90° Fahr. is four times as great as in air at 50° Fahr. ; and 

 the consequence of its being loaded with vapour at the higher temperature is a 

 slight obstruction of the passage of the sun's rays ; and were it not that they 

 strike in Southland with a somewhat greater obliquity than in Sydney, and 

 thus traverse a greater mass of air, it is possible that these high readings 

 of the black bulb thermometer would more frequently happen in the southern 

 parts of the South Island. 



Dr. Hooker, in his observations on the climate of the Himalayas, states 

 that at a height of 10,000 feet at 9 a.m. in the middle of summer the ther- 

 mometer mounted to 132° Fahr. in the sun when the temperature of the air 

 was 32° Fahr., a difference of 100°. This difference, no doubt, would have 

 been much greater had the black bulb been protected from currents of the 

 surrounding cold air, and had it laid on a bed of black cotton-wool. Tyndall 

 quotes this for the purpose of showing that the extraordinary difference of 100° 



only be accounted for by the sun's rays passing through air almost devoid 

 of aqueous vapour as through a vacuum. Dr. Hooker found the same 

 extraordinary difference on the plains of India, because of the dryness of the 



; but no such result had been found in Calcutta, where the heated 

 atmosphere is surcharged with aqueous vapour. Tyndall goes on to say that 

 he himself " never, under any circumstances, suffered so much from heat as in 

 descending on a sunny day from the Corridor to the Grand Plateau of Mont 

 Blanc. The air was perfectly still, and the sun literally blazed 

 and his friend. Though hip deep in snow, still the heat was unendurable." 



What I contend for is, that in high latitudes the air does not contain the 

 same quantity of aqueous vapour as in warmer latitudes. That the presence 

 of aqueous vapour interferes in a slight degree with the passage of solar heat 



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