SCIENTIFIC NEWS. 309 



moon, with precision. Mr. B. likewise proposes a new me- 

 thod of determining the moon's node. 



Mr. Biot, before his first journey into Spain, had deter- Refraction of 

 mined by nice experiments the refracting power of the air gpj^ m ° ot af . 

 and of gasses, which he found to differ very little from what f cteri by 



Mr. Delambre had inferred from his astronomical obser- a( i UL,ous va " 



pour. 

 yations combined with those of Mr. Piazzi. It is well 



known, that refraction varies with the state and temperature 

 of the atmosphere; and astronomers have long applied two 

 corrections, one from the height of the barometer, the other 

 from that of the thermometer. Since the introduction of 

 the hygrometer, it has been questioned, whether this ought 

 not to be employed for a third correction. During near a 

 month, that Mr. Delambre spent in the steeple of Bois- 

 commun, at a time when severe frosts more than once suc- 

 ceeded very damp mists, he endeavoured to ascertain, whe- 

 ther the variation of the hygrometer were attended with any ( . 

 change in terrestrial refraction, and found not the least in- 

 dication of such a change. Mr. Laplace had made the im- 

 portant remark, that the refractive powers of air and the ■ 

 vapour of water, at equal degrees of elasticity, differed very 

 little; but the question was of sufficient importance in 

 astronomy, to be brought to the test of direct experiments. 

 This Mr. Biot has undertaken. He first ascertained the 

 effect of vapour alone. By means of potash lie. dried the 

 warm air included in his prism, while that without was load- 

 ed with all the natural moisture of the atmosphere. The 

 pressure of these two airs, indicated by a barometer within, 

 and another without, was not the same ; the difference being 

 equal to the tension of the atmospheric vapour. The devi- 

 ation of the luminous ray in the prism then gave the refrac- 

 tion produced by the vapour ; and this never differed from 

 what would have been produced by air alone at a similar 

 temperature more than x a few tenths of a second. The 

 mean was O'lo". Hence Mr. Biot infers, that the refrac- 

 tion produced by vapour in the atmosphere may safely be 

 neglected in astronomy. 



[Certain observations by some of the members of the . 



Asiatic Society at Calcutta however lead to a different 

 conclusion.] 



Mr. 



