108 SCIENCE IN SHORT CHAPTERS. 



and solidification of the lava-forming liquid that is, when the 

 formation of new craters had ceased in any given region ; 

 hence they would extend across the minor lateral craters 

 formed by outbursts from the sides of the main cone, in the 

 manner actually observed. 



The fact that the bottoms of the great walled craters of the 

 Moon are generally lower than the surrounding plains must not 

 be forgotten in connection with this explanation. 



I will not venture further with the speculations suggested by 

 the above -described resemblances, as my knowledge of the 

 details of the telescopic appearances of the Moon is but 

 second-hand. I have little doubt, however, that observers 

 who have the privilege of direct familiarity with such details 

 will find that the phenomena presented by the cooling of iron 

 cinder, or other fused silicates, are worthy of further and more 

 careful study. 



CHAPTER XV. 



NOTE ON THE DIRECT EFFECT OF SUN-SPOTS ON TERRESTRIAL 

 CLIMATES. 



PROFESSOR LANGLEY determines quantitatively the effects 

 respectively produced by the radiations from the solar spots, 

 penumbra, and photosphere upon the face of a thermopile, and 

 infers that these effects measure their relative influence on 

 terrestrial climate. 



In thus assuming that the heat communicated to the ther- 

 mopile measures the solar contribution to terrestrial climate, 

 Professor Langley omits an important factor viz. the amount 

 of heat absorbed in traversing the earth's atmosphere ; and in 

 measuring the relative efficiency of the spots, penumbra, and 

 photosphere, he has not taken into account the variations of 

 diathermancy of the intervening atmospheric matter, which 

 are due to the variations in the source of heat. 



Speaking generally, it may be affirmed that the radiations of 

 obscure heat are more largely absorbed by the gases and vapors 

 of our atmosphere than those of luminous heat, and the great 

 differences in the mere luminosity of the spots, penumbra, and 

 photosphere justify the assumption that the radiations of a 



