RADIANT SEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
349 
Lucerne, that the sun’s heat was always more transmissible through water and alum 
in the morning than at mid-day. I have too much confidence in the able experi¬ 
menters here named to think any of them wrong. How then is the discrepancy between 
them to be accounted for ? I think in the following way. What is called the glow 
of the Alps varies greatly with the quantity of suspended matter in the air. When 
pronounced, it shows that the more refrangible constituents have been in great part 
removed from the sun’s rays. The proportion of the less refrangible rays in the 
total radiation is augmented in this way, the relative transmissibility of the heat 
being diminished. It was, I would suggest, heat that had its character impressed 
upon it in this way by scattering, and not by absorption, that yielded the result 
obtained by M. Soket. 
Whatever may be the value of this explanation, one result of great interest to me 
was established by the two French experimenters. Simultaneous observations were 
made by them on the summit of the Eigi and at Lucerne, the vertical distance between 
both stations being 4,756 feet. Within this stratum 17T per cent, of the solar rays 
was absorbed. 
Experiments were made at the same time at both stations on the perviousness 
of water to the solar rays. If, as I contend, a vapour and its liquid absorb the same 
rays, the withdrawal of 17 per cent, of the radiation by aqueous vapour must render 
the residual heat more transmissible by water. This is precisely what the French 
experimenters found it to be. “Through a glass trough 0*08 of a meter in length, 
and full of water, the rays on the Eigi passed in the proportion of 685, and at Lucerne 
in the proportion of 730, per 1,000 of the incident heat.” 
Magnus was so convinced of the impotency of aqueous vapour to arrest radiant 
heat, that in reference to various meteorological phenomena, where the action I had 
ascribed to it offered a satisfactory explanation of the facts, he put in its place mist or 
haze, the existence of which he assumed, even when neither mist nor haze was visible. 
There are various passages in the Essay on Dew which it would be difficult to 
reconcile with this assumption; for they show that even visible atmospheric turbidity 
has by no means the influence which Magnus ascribed to it. 
Thus on the 7th of January, 1814, Wells observed “a little after sunset” a refrige¬ 
ration of 8°, at a time when some parts of the sky were covered with clouds, and the 
lower atmosphere a little obscure.'"' On another evening, “ when the atmosphere was 
neither very clear nor very still ” a difference of 14~° was observed between the tem¬ 
peratures of air and swan down. Wells also observed a refrigeration of 5° when the 
sky was thickly covered with high clouds. A very definite observation in regard to 
haze was made on the 21st of January, 1814. The air at this time was “a good deal 
hazy, t Notwithstanding this, the temperature of swansdown placed on snow was 
13 g lower than that of the air 4 feet above it. Thus, if other circumstances be 
favourable, that is to say, if the air be dry, even a visible haze does not prevent 
* Essays, p. 174. f IbicL, p. 176. 
