536 Dr. Herschel's Experiments on the solar, and 



which the prism, by their different refrangibility, scatters the 

 rays of heat ; and the ordinates to A Q will nearly express the 

 proportional elevations, which a set of equi-changeable thermo- 

 meters would experience, when placed in the different situa- 

 tions of these ordinates. 



Plate XXI. Fig. 1. A, B, is the box which holds the two 

 thermometers, No. 1 and No. 5. C is the board which contains 

 the transmitting holes, the slip of wood for supporting the 

 glasses, and the perpendicular pin for adjusting the angle. 

 D, E, are the boards joined together by hinges. F is a slip of 

 mahogany screwed to E. G is the spring to confine the 

 slip F; which will keep the board D up to any angle less 

 than go degrees. 



Fig. 2, is the cover for shutting the transmitting holes. 



Plate XXII. Fig. 1, is the screen, which may be elevated, 

 by the usual contrivance of springs at the back, to any required 

 height, so as to permit the rays of the sun to pass through the 

 opening in the middle, and to fall upon the transmitting holes 

 of the box A, B, in Plate XXI. 



Fig. 2, is a second upper part of the box A, B, in Plate XXI. 

 The first upper part being screwed off, this is to be put on in- 

 stead of it, when experiments with liquids are to be made. It 

 contains, as before, the two transmitting holes, the slip of wood, 

 and the pin ; and it has moreover a small bracket fastened un- 

 der one of the holes, on which the tube containing the liquid to 

 be tried may be laid. 



Fig. 3, is a third upper part to the box A, B, of Plate XXI. It 

 contains two small holes, for transmitting prismatic rays to the 

 two thermometers A and B, which must now be put into the 

 box, instead of No. 1 and No. 5. The parallel lines a, b, inclos- 



