French National Institute. 497 



lias's experiments, they conjecture that it is really of a me- 

 tallic origin. 



An additional account of M. De Luc's atmospherical 

 electroscope" was read, and also some illustrations of his 

 theory of meteorology, developing his opinions of the origin 

 of repeated thunder-claps, clouds, hail, &c, and other me- 

 teorological phsenomena. The author accounts for the rapid 

 fall of the barometer previous to a thunder-storm, by sup- 

 posing the existence of some unknown light fluid which as- 

 cends in columns at such times. This supposed fluid in his 

 opinion effects various other purposes of atmospherical phae- 

 nomena. 



The Society then adjourned during the long vacation till 

 Thursday the 9th or November next. 



FRENCH NATIONAL INSTITUTE. 



Analysis of the Labours of the Class of Mathematical and 

 Physical Sciences of the French Institute, for the Year 1S0J. 



MATHEMATICAL DEPARTMENT*. 



Astronomy.. — The French" astronomers, who are now in 

 possession of excellent instruments and methods of singular 

 perfection, have not allowed any opportunity to escape of 

 practising upon these instruments and these methods all 

 the amelioration which reflection aided by long practice can 

 suggest. There were grounds for supposing, thai in the 

 construction of telescopes all possible combinations had 

 been exhausted. In fact, the great mirror is necessarily con- 

 cave, in order to collect under one and the same point all 

 the rays of light which it reflects ; but the second mirror 

 may be concave, as in Gregory's telescope, plain as in New- 

 ton's, or convex as in Cassegrain's ; in short, we may sup- 

 press this second mirror as proposed by Lcmaire, and so 

 happily accomplished by M. Herschel. 



Instead of these four plans, all of which have their ad- 

 vantages and disadvantages, M. Burckhardt has proposed to 

 substitute a fifth, which should have in addition all the 

 merit of facility and of convenience. His small mirror is 

 plain like Newton's : instead of placing it obliquely to the 



* Drawn Op by M. Delambre, Secretary. 



Vol. 33. No. 134. June 180Q. I i focus 



