Astro7iomy .—Barometer . 67 



thinking it is too pregnant with difficulties for beginners. He 

 discusses in particular every curve of the second degree, and 

 afterwards shows their analogy with eacli other. Among the 

 curves of the higher orders, he treats in detail the cirsoid 

 and conchoid ; and, passing to the transcendent cnrves, gives as 

 examples th^ cycloid, epicycloid, and hypocycloid. 



His astronomy commences by an introduction perhaps too 

 short for beginners : a few pages are only devoted to the chief 

 phaenomena of the heavens, such as the diurnal motion, the ap- 

 parent motion of the sun, the determination of the equinoctial 

 points, &c. These observations lead to ^ description of the 

 horizon, the equator, and the ecliptic, on which all spherical 

 astronomy depends. 



In theoretical astronomy, the author treats of eclipses at very 

 great length, and so as fully to attain his object. The use of 

 the eclipses of the moon in the determination of geographical 

 distances, might perhaps have been better placed after the de- 

 tails which he subsequently gives on the subject of the figure of 

 the earth. 



M. Schoen thinks that the mean barometrical height ought 

 to be smaller under the equator than in the temperate zones, 

 considering that the gravity is there less. Nevertheless, ac- 

 cording to the known laws of hydrostatics, the absolute weight 

 of the air ought to be equal over the whole surface of the sea, 

 abstraction being made of the temperature : thus, in oider that 

 an equilibrium may subsist, the column of mercury must neces- 

 sarily be higher in places where the weight is not so intense. 



The variations of the barometer, caused by the increase and 

 diminution of the temperature at different heights, are explained 

 in a novel manner which deserves the attention of men of science. 



The laws of Kepler are only exhibited in a historical point of 

 view, since they may be inferred from the preceding part of the 

 work. M. Schoen gives an indirect solution of the problem of 

 the mean anomalv and the true anomaly. 



The article in which the author treats of comets would have 

 been more complete, if he had given a method for calculating the 

 orbit of these extraordinary bodies by means of their apparent 

 course. Lalande, Laplace, or Olbers, could have furnished him 

 with the requisite materials. Tlie article on perturbations is 

 not very extensive, but it is sufficient to give a very correct idea 

 of this remarkable ))hKMomenon. The Appendix contains the 

 history of the Calendar, the comparison of the aara of Nabono- 

 sor with ours ; and, finally, the ordinary principles which are 

 necessary for the dr- wing up of our calendar. Upon the whole, 

 the work is more adapted for those who have gone through a 

 course of astronomy already than for begiimcrs. 



E 2 A great 



