Radiant Heat, and its Conversion tlierehy into Sound, 521 



Experiments were made at the same time at both stations 

 on the perviousness of water to the solar rays. If, as I con- 

 tend, a vapour and its liquid absorb the same rays, the with- 

 drawal 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 metre 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 1000 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 refrigeration 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 atmo- 

 sphere was neither very clear nor very still," a difference of 

 14^° was observed between the temperatures 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"f. Notwithstanding this, the temperature of swan- 

 down placed on snow was 13^° 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 powerful refrigeration. I close these references to 

 mist and haze by mentioning a most striking observation 

 made by Wells on^the 1st of January, 1814. " I found," he 

 says, " during a dense fog, while the weather was very calm, 

 a thermometer lying on grass thickly covered with hoar frost 

 9° lower than another suspended in the air 4 feet above the 

 former"!. Here, as before, low temperature implies scanty 

 vapour, the absence of which enabled the grass to pour its 

 heat even through the interstices of a dense fog §. 



* Essays, p. 174. f Ibid. p. 176. X Ibid. p. 158. 



§ Mr. Glaisher moreover has found differences of from 10° to 12° 

 between grass and air " at times when the sky has been free from clouds 

 but not bright, haze and vapour being prevalent " (Phil. Trans. 1847, 

 p. 145). 



