Intelligence and Miscellaneous Articles. 315 



local deflections at Punno, Damargida, Kalianpur, and Kaliana. Then 



t' — 1= — 1"*50, *" — *'= +3-61, t'" — t"= -4"-20. 



But by the last paragraph, 



* + *' + *" + *"' = _0". 05 . 

 4 



These four equations give 



t=0"'S2, t'=-l"'\8, t" = 2"'43, t'"= — \"-77 



for the local deflections at the four stations of the great arc from 

 Cape Comorin to the Himalayas. They are very small quantities 

 compared with the local deflections which calculation gives for the 

 combined effect of the Himalayas and the ocean (Figure of the Earth, 

 p. 149), viz. 



+ 22"-21, + 17"'23, +21"-05, 34"-16. 



This shows that the variations of density in the crust below must 

 very nearly compensate for the effect of the visible causes existing 

 in the mountains and ocean. It follows that the concluding section 

 of my paper in No. 64 of the Royal Society's ' Proceedings,' regard- 

 ing the constitution of the earth's crust, remains unaffected by the 

 revise of my method of applying the principle of least squares. 



I am, yours faithfully, 



John H. Pratt. 



ON THE SPECTRUM OF AQUEOUS VAPOUR. BY M. JANSSEN. 



I have the honour to communicate to the Academy the discovery 

 of a new optical property of aqueous vapour, a property which would 

 appear to lead to important results in celestial physics and in me- 

 teorology. The optical study of this vapour shows that it possesses 

 an elective power of absorption for light, or, in other words, that this 

 vapour produces dark lines and bands in the spectrum of a luminous 

 ray which traverses a sufficient thickness of it. But before enter- 

 ing into the detail of my observations, I shall request permission to 

 give an abridged account of the researches which have led to the 

 present investigation. 



We know that the illustrious Brewster discovered in 1833 what 

 we call atmospheric or telluric rays of the solar spectrum. Brewster 

 had observed that when the sun was near the horizon, its prismatic 

 image became enriched by new dark bands. This fact, in connexion 

 with others of the same kind — that is to say, the action of nitrous gas 

 and others which produce dark bands in the spectrum of a luminous 

 beam which has traversed them — had led the English physicist to the 

 extremely correct idea that our atmosphere might act like nitrous 

 gas, and thus give rise to the dark bands observed when the sun is 

 in the horizon ; Brewster even had the idea that all the lines of the 

 solar spectrum might be explained in the same manner. This 

 beautiful conception could not be demonstrated completely. In 



