[BARNES] ANCHOR-ICE FORMATION lOl 



conjecture from having overlooked tlie more complete and sudden in- 

 version of the hydrostatic order that takes place over stones than ovei 

 mud; which last is deposited only in places whore the water has a 

 stiller and more equable motion. In such places the ground-gru ia 

 later in forming, and therefore is more rarely seen; and it is doubtful 

 whether Mr. ]\IcKeever had a proper opportunity for noticing it in 

 them. 



But to return to the main point vrhich we have here to main- 

 tain in opposition to the reasoning of M. Arago, the radiation of heat 

 through a body of water. When we construct an achromatic object- 

 glass for a telescope, it does not the less remain a burning-lens when 

 we have included in it a transparent fluid, and no experiment has 

 proved that were the fluid water the case would be altered. We are 

 aware of the danger that has been incurred setting fire to an apart- 

 ment by an ornamental glass globe filled with water, and placed in 

 the sun at a window. But as I cannot particularly refer to circum- 

 stances of time and place of the cases now mention^ed, I made an 

 experiment on the subject with such apparatus as I could find readily 

 ai hand, having no access to better in a remote country place.- In a 

 room, of which the temperature was 50° Fahr., a semiglobular tumbler 

 filled with water, containing about a pint and a half, Avas placed inside 

 n window, in the rays of the low but clear winter sun. The bulb 

 of a thermometer, which had been previously placed in a similar situ- 

 ation till it rose and remained steady at 61°, was shifted into the 

 brightest part of the fan-^shaped focus of rays, into which the light 

 was refracted through the tumbler. In this position it was raised in 

 four minutes to 72°. It was again shifted into the unconcentrated 

 ra}"s passing through the window, when it fell, but more slowly than 

 it had risen; and the experiment was repeatedly renewed with similar 

 results, leaving no doubt that the heat, like the light, radiated through, 

 and was refracted by the water. If the fact is so in regard to the 

 radiation of heat through a mass of water four or five inches thick, 

 where ought we to set the limits of thickness of the mass through 

 which it cannot pass ? Obviously, only where the thickness is so great, 

 that the aggregation of the fluid, and of its minute impurities, prevents 

 the transmission of light, as in the deeps of the sea, but not within 

 the ordinary depths of our clear streams. 



Of the effect of radiation in cooling down the surface of the 

 ground, and substances placed upon it, during 'a clear sky, we cannot 



' It may seem absurd to have had recourse to experiment in a case so 

 plain; but the procedure seemed, at the same time, indispensable, to meet' 

 reasonings promulgated with the authority of such a distinguished name. 



