IS 13.] Imperial InstUute of France, 23? 



instrument which, by means of a formula of M. le Comte 

 Lagrange^ reduces quickly an apparent distance of the moon 

 from the sun to the true distance. A prism of rock crystal gives 

 the double image of the moon under a constant angle of 30^ 

 By the circular motion of this prism behind the transparent part 

 of a small mirror, we obtain the correction of the combined 

 effect of parallax and refraction. A simple proportion is suffi- 

 cient when we have observed two distances of the moon from 

 the sun, and when in that double operation we have brought in 

 contact the two images of the jnoon successively : the difference 

 between these two distances compared with that of 30^ wull give 

 that which results from the variable effect of refraction and pa- 

 rallax. The method is very ingenious. Experience must be 

 consulted to determine its precision. 



In a seventh memoir M. Rochon has given a new process to 

 know by the mean height of a man the distance at which that 

 man is irom the eye of the observer. It is obvious that this 

 method cannot be rigorous, but it may have its utility in the 

 operations of military tactics. The instrument is very simple. 

 No use is made in it of the contact of two images. It is suffi- 

 cient that the feet of the first image appear in the same horizontal 

 line with the head of the second^ of which one can judge almost 

 as exactly as of contact. 



The last memoir of M. Rochon has for its object to render sea 

 water potable. He had directed his attention to this object many 

 years ago, but had suspended his experiments when he under- 

 stood that Meusnier Vi/as engaged in solving the problem. The 

 method of M. Roclion consists in a large alembic, in which an 

 imperfect vacuum may be formed, sufficient to make water boil 

 at 50° Reaumur (144-5° Fah.). 



From the preceding statement it is obvious that M. Rochon 

 endeavours every way to be useful, and that his zeal suggests to 

 him resources as varied as the objects to which he applies them. 



Astronomy. 



M. Bouvard announced to the Class on the 3d of August the 

 discovery, which he had made on the 1st of that month, of a 

 small comet in the constellation Lynx, At the end of the 

 meeting a letter was received from M. Blanfrain, Director of the 

 Observatory of Marseilles, communicating three observations on 

 the same comet, which had been observed at Marseilles ten 

 days sooner by M. Pons, Keeper of the Observatory. This 

 comet was small. The observations, at least at Paris, have been 

 few and difficult. MM. Bouvard and "Nicolet have calculated 

 its orbit^ vvhieh does not resemble that of any of those before 

 known. 



Astronomers neglect no occasion to complete the tables in 



