French Naiiofial Jnstihit'e. 7 1 



be good, and we may concliKJe from it the place of the two 

 nodes. They ought to difler ISO degrees, the known mo- 

 tion excepted of the node, which we must take into account, 

 This difference may serve for correcting the latitude of the 

 star : but even that is unnecessary ; for, having the node, (fof 

 the two errors act in a contrary direction,) the mean between 

 the two determinations of one and the same node will be the 

 position which this node had at the moment of the two ob- 

 servations, however distant from each other. This method 

 is therefore general and complete ; but the opportunities 

 of putting it in practice are unfortunately rare. On con- 

 sulting the annals of astronomy, M. Burckhardt has only 

 found two observations of the Virgin's Spike, and four of 

 Regulus, which are applicable. 



M. Biot, previous to his first journey to Spain, had de- 

 termined by precise and delicate experiments the refrangent 

 power of the air and of gases, and this he found to be 

 very little diflerent from what M. Delambre concluded from 

 his astronomical observations combined with those of M. 

 Piazzi. We know that the refractions vary with the state 

 and temperature of the atmosphere ; and for a long period 

 astronomers applied to the mean quantities two correc- 

 tions, one depending upon the heat of the barometer, and 

 the other upon the degree marked by the thermometer. 

 Since meteorology was enriched with a third instrument; 

 which serves for measuring the degrees of dryness and hu- 

 midity in the air, astronomers were uncertain if the hygro- 

 meter could furnish a third correction.' Some trials had 

 been already made, which had given nothing accurate. For 

 nearly a month which M. Delambre spent in the atecple of 

 Bois-communj at a time when strong frosts had more than 

 once succeeded to very damp fogs, he sought to ascertain 

 if the variations of the hygrometer produced any change! 

 in the terrestrial refractions, and he never found the least 

 indication of it. The author of the Mdcankine Celeste had 

 n)adc the important remark, that, in point of equal elasticity,' 

 the refrangent powers of the air and of the vapour of water 

 would only differ by a very small quantity ; but the question 

 j;o essentially interested astrohoniy, that this truth, already so 



E 4 probable, 



