372 NOTES OF A NATURALIST. 



and still less with the hypothesis of Laplace and Biot. In 

 February, 1864, Count Paul de St. Robert, of Turin, communi- 

 cated to the riiilosophical Magazine a short paper, in which he 

 showed the incompatibility of Mr. Glaisher's results with the 

 ordinary formulae for the reduction of barometric observations, 

 and proposed a new formula based on a law of decrement of 

 heat based upon Mr. Glaisher's tables. In the following June, 

 M. de St. Robert published in the same journal a further paper, 

 in which, still accepting Mr. Glaisher's results as accurate, he 

 investigated the subject in a masterly manner, as well with 

 reference to the measurement of heights, as in its connection 

 with the determination of the amount of atmospheric refraction. 

 The formula proposed by M. de St. Robert, and the tables 

 subsequently published by him for its adaptation to use, 

 appearing to be at once the most accurate and the most 

 convenient, have been adopted by myself and by many other 

 travellers ; * but it is evident that their value depends on the 

 correctness of the results, above referred to, deduced by Mr. 

 Glaisher, and their conformity with observation in mountain 

 countries. 



Before we inquire into the conclusions to be drawn from 

 observation, it may be well to point out how incomplete is our 

 knowledge of the physical agencies which regulate the distri- 

 bution of temperature in the atmosphere. 



The primary source of temperature is solar radiation, and its 

 effect at any given point on the earth's surface depends on the 

 absolute amount of heating power in the sun's rays, irrespective 

 of absorption, commonly designated the solar constant, and on 

 the proportion of heat which is lost by absorption in passing 

 through the atmosphere. The temperature of the air at any 

 point will, in the first place, depend on the amount of solar 

 radiation and of heat radiated from terrestrial objects directly 

 absorbed, and next on the heating of the strata near the surface 

 by convection. The amount of heat received from the sun, 

 directly or indirectly, varies of course with the sun's declination 



* It is remarkable that there is no reference to the investigations of 

 M. de St. Robert, and the formula deduced from them, in the article 

 on the "Barometrical Measurement of Heights," in the new edition of 

 the Encyclopccdia Britannica. 



