THE MOON. 



307 



that edge just for the same reason and in the same manner as the ray from the 

 coin is bent over the side of the bucket so as to render the coin visible not- i 

 withstanding the opacity of that side. 



Fig. 4. 



This reasoning leads to the conclusion that as the moon moves over the face 

 of the firmament, stars will be continually visible at its edge which are really 

 behind it if it have an atmosphere, and the extent to which this effect will take 

 place will be in proportion to the density of the atmosphere. 



The magnitude and motion of the moon and the relative positions of the stars 

 are so accurately known that nothing is more easy, certain, and precise, than 

 the observations which may be made with the view of ascertaining whether 

 any stars are ever seen which are sensibly behind the edge of the moon. Such 

 observations have been made by the most skilful astronomers, and no such ef- 

 fect has ever been detected. This species of observation is susceptible of 

 such extreme accuracy, that it is certain that if an atmosphere existed upon 

 the moon a thousand times less dense than our own, its presence must have 

 been detected. 



But what is an atmosphere a thousand times less dense than ours ? Our at- 

 mosphere supports by its pressure a column of thirty inches of mercury in the 

 barometer. One a thousand times less dense would not support so much as 

 the thirtieth of an inch ; in short, it may be considered as proved that there 

 does not exist upon the moon an atmosphere as dense as is found under the re- 

 ceiver of the most perfect air-pump after that instrument has withdrawn from 

 it the air to the utmost extent of its power. In fine, it may be considered as 

 demonstrated that there is no air upon the moon. 



THE PHYSICAL QUALITIES OF MOONLIGHT. 



It has long been an object of t inquiry among philosophers whether the light 

 of the moon has any heat, but the most delicate experiments and observations 

 have failed to detect this property in it. 



A thermometer of extreme sensibility, called a differential thermometer, was 

 the instrument applied to this inquiry. Let E and F be two thin glass bulbs 

 connected by a rectangular glass tube E A B F partially filled with a liquid to 

 the level. Let the bulbs E and F contain air. If the bulb F be exposed to 

 any source of heat or cold different from E, the air within it will expand or 

 contract., and the liquid in F B will fall or rise. This instrument has such ex- 

 treme sensibility that it is capable of rendering manifest a change of tempera- 

 ture amounting to the five hundreth part of a degree. The light of the moon 

 was collected into the focus of a concave mirror of such magnitude as would <[ 

 have been sufficient, if exposed to the sun's light, to evaporate gold or platinum. ] 

 The bulb of the differential thermometer was placed in its focus so as to re- i[ 



