OF ARTS AND SCIENCES. 133 



cal features, that the rocks of which the surface of the moon is made 

 up are of volcanic character. 



Lunar Radiation. 



While the light received from the moon has been frequently meas- 

 ured by many different observers, the more important problem of its 

 dynamic radiation has been solved by but a single person. Observers 

 previous to Langley can pretend to so little accuracy in their results, 

 that our knowledge of lunar radiation may be said to rest upon his 

 authority alone. This condition does not arise from any lack of in- 

 terest in the subject, — on the contrary the problem has always been a 

 fascinating one, — but is due to the fact, that, previous to the invention 

 of the bolometer, no instrument existed capable of dealing accurately 

 with so small an amount of heat as the moon affords. Great interest, 

 therefore, cannot fail to attend the results of careful observation, made 

 with competent instruments, and tending to increase our knowledge of 

 the subject. 



The first trial of the new thermograph on the moon was made on 

 January 27, 1888. The apparatus, tested at that time, has been regu- 

 larly employed since, and has performed so satisfactorily that it has 

 scarcely been altered from its original form. The arrangement of the 

 thermograph for the measurement of lunar radiation is this. An 

 oblong box of pine wood carries at one end a silvered glass mirror, 

 0.196 metre in diameter, and of one metre focus. This mirror is ad- 

 justable by screws passing through the end of the box and bearing 

 upon its back. Near the other end of the box is fixed a grooved 

 block, upon which the thermograph may be placed and secured. The 

 block is also provided with adjusting screws, so that the thermograph 

 may be made to point exactly at the centre of the mirror. The mirror 

 is inclined, so that the moon's image is thrown into the opening of tin- 

 thermograph, and brought to a focus upon the strip. The whole 

 arrangement is that of a Herschel's telescope with the thermograph in 

 place of an eye-piece. 



To secure accuracy in pointing, the opening in the thermograph is 

 made of such a size as just to receive the cone of rays from the large 

 mirror, and, a rim of white paper being glued about the opening, the 

 slightest departure from a central position of the cone of rays is an- 

 nounced by the appearance of light upon this paper. A window oi 

 plate glass has its place in the side of the box, just in front of tin- 

 position occupied by the thermograph, and a person looking through 

 this can readily direct the image of the moon into the orifice of tin- 



