RADIATION OF HEAT. 



415 



scale is attached, divided into 100, and the liquid contained in the tube is so 

 disposed, that it stands in the graduated leg opposite that point of the scale 

 which is marked 0, when both bulbs are exposed to the same temperature. 

 The glass ball attached on the leg of the instrument which bears the scale, is 

 called the focal ball. Dry air is contained in the balls above the sulphuric 

 acid, which, not being vaporizable, does not affect the pressure of the air above 

 it by its vapor. 



If this instrument be brought into a warm room, the air contained in both 

 bulbs is equally affected by the increase of temperature, and tkerefore no change 

 takes place in the position of the liquid ; and whatever changes the tempera- 

 ture of the apartment may undergo, for the same reason, produce no effect on 

 the instrument. Suppose, however, that the focal ball F is submitted to the 

 effect of heat, from which the ball E is free ; then the air in F will acquire a 

 greater degree of elasticity, while the air in E maintains its former pressure ; 

 the liquid in the leg F B will, therefore, be pressed downward, until the in- 

 creased space obtained by the air in F, and the diminished space into which 

 the air in E is pressed by the ascent of the liquid in A E is such, that the pres- 

 sure of the air in the two balls, by diminishing that of the air in F and increas- 

 ing that of the air in E, acquires a difference which is equal to the weight of 

 the column by which the height of the liquid in A E exceeds the height of the 

 liquid in B F. In fact, the least attention to the instrument will show, that the 

 difference of the heights of the columns of liquid in the two vertical tubes, 

 will represent the difference between their pressures of the air contained in the 

 two bulbs. It is from this property of indicating, not the absolute temperatures, 

 but the difference of the two adjacent points, that the instrument has received 

 its name. 



Let M M', fig. 6, be two concave mirrors, placed face to face, at the distance 

 of ten or twelve feet, having a certain form called parabolic, the property of 

 which we shall now describe : If the flame of a candle, or any other source 

 of light, be placed at a point/, called the focus of the mirror M, the rays of 

 light which proceed from it in every direction, and strike on the concave surface 

 of the mirror M, will be reflected in parallel lines toward the mirror M'. . When 

 these parallel rays encounter the surface of the reflector M', they will be again 

 reflected by it, in lines which all converge to the same point/', which is the 

 focus of M'. Now, instead of a luminous flame, let amadou, gunpowder, or 

 other matter easily inflammable, be placed in the focus/ and place a red-hot 

 metallic ball in the other focus /'. In a few minutes the amadou or gunpow- 

 der will be inflamed or exploded by the heat radiated by the ball and collected 

 at (he point fby the reflectors M M'. 



Fig. 6. 



Bui to prove that the rays of non-luminous heat are similarly reflected, let 

 the red-hot ball be removed, and a hollow ball of metal, filled with boiling wa- 

 ter, be substituted for it at/' ; let the focal ball of a differenti^ thermometer be 







