1883.] 



Atmospheric A bsorption. 



81 



wind blowing, and also at a high altitude on a dry day, these absorp- 

 tions nearly if not quite disappear. If we examine photographs 

 taken when the air is nearly saturated with moisture (in some form 

 or another) we have a spectrum like fig. 1. Except with very pro- 

 longed exposure no trace of a spectrum below A, 8330 can be pho- 

 tographed. Fig. 2 shows the absorption bands, where there is a dif- 

 ference of about 3° between the wet and dry bulb, the latter standing 

 at about 50°. It will be noticed that the spectrum extends to 

 the limit of about A, 9420, when total absorption steps in and 

 blocks out the rest of the spectrum. Fig. 3 shows the spectrum 

 where the difference between the wet and the dry bulb is about 6°. 

 Figs. 5 and 6 show the absorption of thicknesses of 1 foot and 

 3 inches of water respectively, where the source of light gives a 

 continuous spectrum; J inch water merely shows the absorption, 

 bands below 9420, It will be seen that there is an accurate co- 

 incidence between these " water bands" and the absorption bands 

 seen in the solar spectrum, and hence we cannot but assume that 

 there is a connexion one with the other. In fact, on a dry day it is 

 only necessary to place varying thicknesses of water before the slit of 

 the spectroscope and to photograph the solar spectrum through them, 

 in order to reproduce the phenomena observed, on days in which there 

 is more or less moisture present in the atmosphere. It is quite easy 

 to deduce the moisture present in atmosphere at certain temperatures 

 by a study of the photographs. There does appear a difference, how- 

 ever, in the intensity of the banded absorptions in hot weather and 

 in cold about up to 50°. In the former they are less marked when 

 the degree of saturation and the length of atmosphere traversed are 

 the same as in the latter. 



The accepted view, we believe, of absorption of vapours is that 

 they give linear absorptions in certain thicknesses, and as the thick- 

 ness increases or the density becomes greater, the lines blacken, new 

 lines appear, and gradually total absorption sets in in the region where 

 the lines are most numerous and close. It is in the range of possibility 

 that the presence of a small quantity of vapour might show itself as 

 a haze over some region of the spectrum ; if, however, the quantity 

 was gradually increased the haze would give place to the lines, and the 

 phenomena just described would be repeated. Suppose several 

 localities of absorption to exist, the absorptive power of the vapour 

 increasing the further the locality was situated down in the infra-red, 

 it might happen that whilst one locality showed only a haze of 

 absorption, one further down might show total absorption, some 

 locality between these two should show linear absorption. 



In the case of the absorptions in the solar spectrum we find a very 

 different state of things existing. A comparison of the photographs 

 taken in London on days of different dryness, and with those taken at 



vol. xxxv. G 



