39 Solar Radiation 



sun's heat radiation, as it reaches the surface of the earth, 

 has to be determined by other means. When it has been 

 ascertained under the most favourable circumstances it does 

 not vary, excepting in the annual cycle of the earth's 

 revolution. The diurnal variation, as shown by registering 

 actinometers, would have a great local importance. Crova, 

 in the long series of valuable observations which he has 

 made since 1875 at Montpelier, has, in fact, put this prin- 

 ciple in practice. 



Very important observations have been made in the 

 neighbourhood of Chamonix by Violle and afterwards by 

 Vallot. The Annales de f Observatoire meteorologique du 

 Mont Blanc contain, in vol. ii., several interesting reports on 

 the results of these observations. They were made simul- 

 taneously at Chamonix and at certain stations on Mont 

 Blanc. The first series of observations was made in 1887 

 on July 28, 29 and 30, and the instruments used were two 

 "absolute actinometers" of Violle (Ann. Chim. Phys. 1879 

 [5], t. xvii.). 



The great advantage of such experiments is that they 

 are made simultaneously at two stations situated at very 

 different altitudes. At the higher of the two the average 

 barometric pressure is 430 millimetres, so that ff of the 

 whole atmosphere are below the observer, and this portion 

 contains nearly all the aqueous vapour. Above him there 

 is a little more than one-half, and that much the simpler 

 and purer half of the atmosphere. In it aqueous vapour is 

 almost absent. The summit of Mont Blanc is 4807 metres 

 and the station at Chamonix is 1087 metres above the sea. 

 The layer of the atmosphere separating them has, therefore, 

 a thickness of 3720 metres, and it can be visited at any 

 point in its thickness. M. Vallot has acquired a personal 

 acquaintance with this layer of air which can only be 

 obtained by devoting a number of years to living in it and 

 observing it. It is this intimate and continuous acquaintance 

 with so large a proportion of the earth's atmosphere that 

 entitles the observations and conclusions of M. Vallot to 

 especially great weight. 



