1815.] Sir Benjamin Thompson, Knt, Count Rumford. 247 



Air 576" 



Hare's fur 1815 



Eider down 1305 



Beaver's fur 1296 



Raw silk 1284 



Sheep's wool 1118 



Cotton wool 1046 



Fine lint 1032 



The finer the fibres were the worse a conductor of heat was the 

 substance. He ascribes this to the attraction between the fine 

 fibres and air, which prevents the air from moving: out of its place, 

 and thus carrying off the heat. In these experimentSj likewise, 

 the radiating power of different bodies is overlooked. 



6. A Method of Measuring the comparative Intensities pf the 

 Light emitted by Luminous Bodies, Phil. Trans. 1794. P. 67. — 

 The method is to make the luminous bodies compared shine on a 

 sheet of paper in the middle of which is placed a small cylindrical 

 body. This cylinder will project two shadows, one illuminated 

 by one of the luminous bodies, the other by the other. The 

 distance of one of the lights is to be varied till the two shadows are 

 of equal intensity. The light emitted by each body is as the square 

 of its distance from the cylinder. Tiiis apparatus our author called 

 a Photometer. It is nearly similar to the method long before 

 employed by Bouguer, of which Count Rumford was probably not 

 aware, as he takes no notice of it. The facts ascertained by Count 

 Jlumford by means of this apparatus were the following : — No 

 perceptible quantity of the light is absorbed during its passage 

 through air. When light passes through fine mirror glass about i-th 

 is absorbed. About -} is lost when light is reflected from a good 

 plane glass mirror. A good Argand's lamp gives as much light as 

 eight wax candles \ inch in diameter. The quantity of oil con- 

 sumed in an Argand's lamp to that consumed in a common lamp to 

 produce the same light is 15 per cent. less. The light of a candle 

 fluctuates much more than that of a lamp. A tallow candle newly 

 snuffed gave out a quantity of light = 100 ; in 29 minutes the - 

 light was reduced to 16. On being snuffed, it recovers its 

 original intensity. The following table exhibits the weights of 

 different substances, which must be consumed in order to produce 

 tlie same quantity of light : — 



Weiglil. 



Bees' wax. A good wax candle kept well snuffed, and "i 

 burning with a clear bright flame J 



Tallow. A good tallow candle kept well snuffed, and") 

 burning with a bright flame J 



Tlie same tallow candle, burning very dim for want of ) ^^^^ 

 snuffing J ^-^^ 



Olive oil burnt in an Argand's lamp 1 10 



