en the terrestrial Rays that occasion Heat. 537 



ing the holes, will direct any coloured rays to the thermometers ; 

 and, by drawing the red rays down to the lower parallels c or d, 

 invisible rays may be brought to enter the transmitting holes. 



Fig. 4, is a fourth upper part to the box A, B, Plate XXI. It 

 contains two large holes, for admitting the rays of the sun to fall 

 upon the objects on Fig. 5. 



Fig. 5, represents two tablets, a,b, united ; they may be covered 

 with any objects that are to be examined ; for instance, a with 

 white paper, and b with black velvet. These tablets, by a pro- 

 per contrivance, are brought under the holes of figure 4, where 

 a button fastens them at the required distance. 



Plate XXIII. Fig. 1. A, B, is the box which holds the thermo- 

 meters. C, D, are the screens, with the transmitting holes in them, 

 opposite the flame of E, the candle. F, is a small weight, stretch- 

 ing a string across the glass, or other object, placed upright 

 against the transmitting hole of the screen D. It may be carried 

 round the screen C, if required, and hold a glass against the 

 hole in it. 



Fig. 2, is the double cover : in putting it on, it must be passed 

 over the candle downwards, against the two transmitting holes. 



Plate XXIV, represents a large screen, with the shelf A, B, 

 carrying four thermometers, No. 1 and No. 5 opposite the 

 transmitting holes, and the other two as represented. When 

 the fire is properly prepared, lift the screen, by taking hold a 

 little above A and B, and set it close to the fire, so that C, D, 

 may touch the bottom of the grate ; then take hold of the ring 

 E, and pull the string far enough out to hang that ring on the 

 hook F : this will open the transmission holes, when the expe- 

 riment is to begin. 



Plate XXV. Fig. 1, is the box containing the thermometers, 



