DEGREES OF HEAT. 131 



the measure of another property in the same way 

 that one weight is the measure of other weights, 

 or one length the measure of other lengths, there 

 is no standard to which we can bring either light 

 or heat, except we make some degree of each 

 which we find constant, as displayed in some sub- 

 stance, the measure of the other's degrees. The 

 variations of light are so very delicate in them* 

 selves, and they are so much confused by the 

 variations of colour, that it is scarcely possible to 

 obtain any contrivance by which light can be 

 made the measure even of itself. 



Various instruments, called photometers, that is, 

 " light measures," have been invented by ingeni- 

 ous men ; but the majority, if not the whole of 

 these, are affected by, and therefore measure 

 heat, and not light ; and thus they are, in truth, 

 thermometers, or heat measures, of more nice 

 construction and greater sensibility than the com- 

 mon ones. 



It is, indeed, exceedingly difficult even to con- 

 template light without having the notion of heat 

 along with it ; and, indeed, we have not much 

 knowledge of, especially great degrees of heat, 

 without light along with it. In poetical language 

 it is not uncommon to speak of " the wan cold 

 moon," and " the cold moonbeams ;" and there 

 is truth as well as poetry in those expressions. 

 It has been mentioned that the red rays of the 

 sun penetrate the most readily into the substances 

 on which they fall ; and the greatest heat, which 

 is at the red end of the spectrum, penetrates still 

 more readily than the red rays. Now, our moon- 

 light really comes from the sun, and is reflected 

 to us from the surface of the moon, just as we 



