420 Chronicles of Science. [ Jiily> 



between the jaws of the spectroscopic-slit, or the spectrum will not 

 be visible. 



Mr, Proctor supplies the elements of his recalculation of the 

 rotation of Mars. The total period taken into account contains 

 640,284,123 seconds, corresponding to 72,232 rotations of the 

 planet, and giving a rotation-period of 24 h. 37 m. 22-786 s. He 

 remarks that Kaiser's period (24 h. 37 m. 22*625 s.) would throw 

 the Kaiser sea so far from the centre of the disc at the epoch of 

 Hooke's observation, that that feature would have been concealed 

 from view by the haze which always hides the planet's hne. Hooke's 

 picture shows the Kaiser sea only 18^ from the centre of the disc. 

 On the other hand, Kaiser's estimate, hitherto undoubtedly the 

 most exact, suffices to show that it was the Kaiser sea, and not 

 the somewhat similar Dawes' strait, that Hooke saw ; for the latter, 

 according to Kaiser's period, must have been on the farther side 

 of the planet at the time of Hooke's observation. The period given 

 by MM. Beer and Miidler (24 h. 37 m. 23 • 8 s.), on the other hand, 

 although accounting for Hooke's obseiTation on the omission of a 

 complete rotation, is shown by Sir W. Herschel's drawings to be 

 incorrect. It is a matter of some interest thus to be able to assign 

 the exact period of a planet's rotation. As om- earth's motion of 

 rotation is slowly diminishing through the moon's action, it may 

 be important, at some far distant epoch, to have in Mars a cos- 

 mical clock of undoubted accuracy. For the comparative smallness 

 of Mars, his greater distance from the sun, the smaUer proportion 

 which the seas on his surface bear to the continents, and the fact 

 that he has no sateUite, all encourage the belief that his period of 

 rotation cannot be affected appreciably by the motion of a tidal 

 wave, as is the case with the earth. 



A paper by ]Mr. Maclear on the subject of the Meteoric shower 

 of November, 1868, is chiefly remarkable for the endeuce it 

 supphes of the wide range over which the display of last November 

 was to be seen. We have already had accounts of the shower as 

 seen in America ; it was well seen in England ; and it now appears 

 that it was seen under favourable circumstances at the Cape of 

 Good Hope. This suffices to show that the part of the meteor 

 system traversed last November differs wholly in character from 

 that which gave bii'th to the more brilliant, but much more short- 

 lived display of November, 1866. 



Professor Cayley gives a simple proof of the property, that if 

 an indefinitely thin shell of uniform density, rounded by t^yo 

 similar and similarly situated ellipsoids, attract a point P on its 

 outer surface, the attraction (assumed to act in the direction of the 

 normal) is equal to twice the attraction of an mfinite plate, 

 the thickness of which is equal to the normal thickness of the shell 

 at the point P. The proof may be thus summarized : — 



