OF ARTS AND SCIENCES. 139 



passage of the reflected rays to the surface of our planet, receive any- 

 thing like the maximum possible radiation. Our atmosphere cuts off 

 a considerable portion of the lunar rays. It has been observed that 

 aqueous vapor is particularly efficacious in stopping them. It may 

 not be uninstructive, as illustrating the minuteness of the quantity of 

 heat that we receive from the moon, to express it in terms of melting 

 ice. It will not be far from the truth if we say that the sun's rays 

 will thaw through an inch of ice in 100 minutes, whence it follows, 

 f'iom our determination, that the rays of the moon will melt the same 

 thickness of ice in 18,456,000 minutes, or 35 years. 



That portion of the solar rays not reflected from the lunar surface 

 is absorbed by it, and radiated at different wave lengths. At what 

 wave lengths we do not know, and whether or not those rays are 

 capable of passing our atmosphere we cannot tell. An idea of the 

 general character of the lunar rays may be gained from experiments 

 with absorbing media. For example, on April 25, 1888, the per- 

 centage of lunar rays transmitted by a plate of quartz was determined. 

 The quartz plate was 5 mm. thick, cut perpendicularly to the axis of 

 the crystal. It was arranged to be drawn before the thermograph by a 

 thread, so that the observations could be made in pairs, alternately 

 with and without the quartz. The results were as follows : — 



Mean of the whole (GO readings) . . . 31.3% 



An attempt was then made to find some artificial source of heat 

 whose radiations would be transmitted by the plate of quartz to the 

 extent, approximately, of 31%. It was discovered that a coil of 

 platinum wire, in a Bunsen lamp, turned as low as possible, very 

 nearly fulfilled the requirement. We may say, then, that in general 

 character the rays from the moon resemble those from an incandescent 

 platinum wire. The plate of quartz used in the above experiments 

 transmits 93.3% of the sun's rays. 



A study of the reflection of the sun's rays from rocks may serve, to 

 some extent, to aid in the lunar problem, and to this end a few experi- 

 ments have been made, after the following manner. 



The thermograph is mounted upon a strip of board, which can ho 

 turned about a pin passing through it and into the table below. 'I he 

 rock under experiment is held in a clamp a short distance in front of 



