on the terrestrial Rays that occasion Heat. 477" 



holes. A small thin wooden partition is run up between the 

 thermometers, to prevent the heat transmitted through one hole 

 from coming to the thermometer belonging to the other. 



The screen is fixed upon a light frame, which fits exactly into 

 the opening of the front of the marble chimney-piece ; and the 

 ends of the frame are of a length which, when the screen is 

 placed before the fire, will just bring the transmitting holes to 

 be 6>i inches from the front bars of the grate. 



A large wooden cover, also plated with iron, shuts up the 

 transmitting holes on the side next to the fire; but may be 

 drawn up by a string on the outside, so as to open them when 

 required. 



Two assistant thermometers are placed on proper stands, to 

 bring their balls to the same distance from the screen as those 

 which receive the heat of the fire ; but removed sideways as far 

 as necessary, to put them out of the reach of any rays that pass 

 obliquely through the transmitting holes. They are to indicate 

 any change of temperature that may take place during the time 

 of the experiment : for, notwithstanding the largeness of the 

 screen, some heat will find its way round and over it ; and this 

 acting as a general cause, its effect must be allowed for. 



117 th Experiment. 



Having tried the apparatus sufficiently to find that the ther- 

 mometers exposed to the transmitting holes would generally 

 receive 20 or more degrees of heat, without differing more than 

 sometimes \ or at most ^ of a degree, I now placed the bluish- 

 white glass of the 24th experiment upon a support prepared for 

 the purpose, so as closely to cover one of the transmitting holes. 

 A small spring, moveable on its centre, is always turned against 



mdccc 3 Q 



