FORMED AT THE BOTTOM OF RUNNING WATER. 339 



have rough surfaces. Thus rocks in large or small masses will become much cooler 

 in consequence of radiation : when the atmospherical temperature is very low, they 

 of course freeze the water which touches them.' It is unnecessary to examine here 

 whether heat radiates through a thick layer of water, as Mr. M'Keever supposes, as 

 the most simple observation is sufficient to overthrow it. Where is the person who 

 has not observed that the strong radiation, which the Irish philosopher admits, would 

 be more plainly manifested, or as completely, in still water than in running water ? 

 but no one has seen a piece of still water frozen at the bottom*." 



But there is nothing more easy of experimental proof than that heat radiates through 

 water, I do not mean, however, to vindicate the reasoning of Mr. M'Keever re- 

 specting the more powerful radiation of it from stones than from mud. His reason- 

 ing respecting that matter is, on his own part, conjectural, to explain the readier 

 formation of gru on a stony or gravelly bottom ; but the gru also forms on a muddy 

 bottom, a fact which M. Arago notices, when he brings the attachment of mud to 

 the under side of the floating flakes as a proof that they have been formed at the 

 bottom. Mr. M'Keever was driven to his conjecture from having overlooked the 

 more complete and sudden inversion of the hydrostatic order that takes place over 

 stones than over mud ; which last is deposited only in places where the water has a 

 stiller and more equable motion. In such places the ground-gru is later in forming, 

 and therefore is more rarely seen ; and it is doubtful whether Mr. M'Keever had a 

 proper opportunity for noticing it in them. 



But to return to the main point which we have here to maintain 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 of setting fire to an apartment by an ornamental glass 

 globe filled with water, and placed in the sun at a window. But as I cannot parti- 

 cularly refer to circumstances of time and place of the cases now mentioned, I made 

 an experiment on the subject with such apparatus as I could find readily at hand, 

 having no access to better in a remote country place-f . In a room, of which the 

 temperature was 50° Fahr., a semiglobular tumbler filled with water, containing 

 about a pint and a half, was placed inside a window, in the rays of the low but clear 

 winter sun. The bulb of a thermometer, which had been previously placed in a 

 similar situation 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 rays passing through the window, when it fell, but more 



* Edinburgh New Philosophical Journal, vol. xv. pp. 132, 133. 



t 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. 



