on the terrestrial Rays that occasion Heat. 44,7 



spring must be sufficiently strong to keep the boards at any 

 angle; and the slip of mahogany long enough to permit an 

 elevation of about 85 degrees. 



In order to see whether all be properly adjusted, expose the 

 apparatus to the sun, and lift up the board which carries the 

 box, till the directing pin throws the shadow of its head on the 

 place where the point is fastened. Then hold a sheet of paper 

 under the box, and, if the thermometers have been properly 

 placed, the shadow of their balls will be in the centre of the 

 rays passing through the transmitting holes to the paper. 



A screen of a considerable size,* with a parallelogrammic 

 opening, should be placed at a good distance, to keep the sun's 

 rays from every part of the apparatus, except that which is 

 under the cover ; and no more sun should be admitted into the 

 room, than what will be completely received on the screen, inter- 

 posed between the window and the apparatus. 



As one of the thermometers is to indicate a certain quantity 

 of heat coming to it by the direct ray, while the other is to 

 shew how much of it is stopped by the glass laid over the trans- 

 mitting hole, it becomes of the utmost consequence to have two 

 thermometers of equal sensibility, -f The difficulty of getting 



* See Plate XXII. Fig. I. 



f The theory of the sensibility of thermometers, as far as it depends on the size of 

 the balls, may be considered thus. Let D, d, S, s, T, t be the diameters, the points 

 on which the sun acts, and the points on which the temperature acts, of a large and a 

 small thermometer having spherical balls ; and let x '. y be the intensity of the action 

 of the sun, to the intensity of the action of the temperature, on equal points of the 

 surface of both thermometers. Then we have s ; S ; ; d 1 : D a , and t ; T : ; 4c? 1 : 4 D\ 

 The action of the sun therefore will be expressed by d 7, x, D 1 x ; and that of the tem- 

 perature by 4 (Py , 4D 1 )'; and the united action of both by x — 4 y X d*, x — 4 y X D* > 

 which are to each other, as d z ; D 1 . Now, the total effect being as the squares of the 



3 M 2 



