754 PHILOSOPHICAL TRANSACTIONS. [ANNO 1 80O. 



As one of iae thermometers is to indicate a certain quantity of heat coming to 

 it by the direct ray, while the other is to show how much of it is stopped by the 

 glass laid over the transmitting hole, it becomes of the utmost consequence to 

 have 2 thermometers of equal sensibility*. The difficulty of getting such is much 

 greater than can be imagined: a perfect equality in the size and thickness of the 

 balls is however the most essential circumstance. When 2 are procured, they 

 should be tried in quick and in slow exposures. These terms may be explained by 

 referring to fire heat ; for here the thermometers may be exposed so as to acquire, 

 for instance, 30° of heat in a very short time; which may then be called a quick 

 exposure: or they may be placed so as to make it require a good while to raise 

 them to so many degrees; on which account the exposure may be called slow. It 

 is true, that we have it not in our power to render the sun's rays more or less 

 efficacious, and therefore cannot have a quick or slow exposure at our command; 

 but a great difference would be found in the heat of a rising, or of a meridian sun : 

 not to mention a variety of other causes, that influence the transmission of heat 

 through the atmosphere. Now when thermometers are tried in various expo- 

 sures, they should traverse their scales together with constant equality ; otherwise 

 no dependence can be placed on the results drawn from experiments made with 

 them, in cases where only a few minutes can be allowed for the action of the 

 cause whose influence we are to investigate. The balls must not be blacked : for, 

 as we have already to encounter the transmitting capacity of the glass of which 

 these balls are made, it will not be safe to add to this the transmitting disposition 

 of one or more coats of blacking, which can never be brought to an equality, 

 and are always liable to change, especially in very quick exposures. 



Transmission of Solar Heat through Colourless Substances. 



Ecrper. 24. — I laid a piece of clear transparent glass, with a bluish-white cast, on 

 one of the holes of the transmitting machine : the faces of this glass are parallel, 

 and highly polished. Then, putting the cover over both holes, I placed the machine 

 in the situation where the experiment was to be made, and let it remain there a 

 sufficient time, that the thermometer might assume a settled temperature. For 



* 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, *, 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 i and 

 let x toy 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 :: d l : d*, and t : t :: 4d* : 4d\ 

 The action of the sun therefore will be expressed by d l x, d 2 x; and that of the temperature by 

 Ad 1 }/, 4D*y; and the united action of both by (x — 4y) x cP, (x — 4y) x d*j which are to each 

 other, as «P : d 9 . Now the total effect being as the squares of the diameters, while x : y remain in 

 their incipient ratio, and the contents of the thermometers being as the cubes, the sensible effect pro- 



d* d 1 11 



duced on the particles of mercury, must be as — : — :: -r : -; that is, inversely as the diameters. The 

 r a* d* a d 



small thermometer therefore will set off with a sensibility greater than that of the large one, in the 



same ratio. — Orig. 



