44$ D r - Herschel's Experiments on the solar, and 



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 two 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 degrees 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 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 com- 

 mand; but a great difference will 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 

 exposures, they should traverse their scales together with con- 

 stant equality ; otherwise no dependance 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 



diameters, while x \ y remain in their incipient ratio, and the contents of the thermo- 

 meters being as the cubes, the sensible effect produced on the particles of mercury, 



d 1 D* 1 1 

 must be as -r, I ™ • • "5 • 7; J that is, inversely as the diameters. The small thermo- 

 d J 1) J a V 



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



same ratio. 



