44^ Dr. Herschel's Experiments on the solar, and 



be added. And a mirror, besides its natural incapacity of sepa- 

 rating the rays of light from the different sorts of heat, scatters 

 them very profusely. But, if we have been scantily provided 

 with materials to act upon rays, it has partly been our own 

 fault : every diaphanous body may become a new tool, in the 

 hands of a diligent inquirer. 



My apparatus for transmitting the rays of the sun is of the 

 following construction.* In a box, 12 inches long, and 8 inches 

 broad, are fixed two thermometers. The sides of the box are 

 2^ inches deep. That part of the box where the balls of the 

 thermometers are, is covered by a board, in which are two 

 holes of \ inch diameter, one over each of the balls of the 

 thermometers ; and the bottom of the box, under the cover, is 

 cut away, so as to leave these balls freely exposed. There is a 

 partition between the two thermometers, in that part of the box 

 which is covered, to prevent the communication of secondary 

 scatterings of heat. Just under the opening of the transmitting 

 holes, on the outside of the cover, is fixed a slip of wood, on 

 which may rest any glass or other object, of which the trans-* 

 mitting capacity is to be ascertained. A thin wooden cover is 

 provided,-^ that it may be laid over the transmitting holes, occa- 

 sionally, to exclude the rays of the sun ; and, on the middle of 

 the slip of wood, under the holes, a pin is to be stuck perpen- 

 dicularly, that its shadow may point out the situation of the 

 box with respect to the sun. The box, thus prepared, is to be 

 fastened upon two short boards, joined together by a pair of 

 hinges. A long slip of mahogany is screwed to the lowest of 

 these boards, and lies in the hollow part of a long spring, 

 fastened against the side of the upper one. The pressure of the 



* See Plate XXI. Fig. 1. f See Fig. 2. 



