ROTATION OF MARS 67 



planet varied so much that no reliance could be placed 

 10 mm It an a mean* of ascertaining whether oar 



' the same from age to age. 



>*ehel considered the planet Mara a more favour- 



experiment than Jupiter. On Mara he 



aaw spots of a different nature : constant and 



determined shape, as well as remarkable colour, show 



them to be permanent and fastened to the body of 



the planet Thcae will ^ivo the revolution of hi* 



to a great certainty, and by a great number 



volutions, to a very great exactness aba" A 



instance, with wl schel was not acquainted, 



materially helped him in his observations on Man. 



The atmosphere on that planet is not nearly so dense 



s, and similar trade-wind belts to those 



on Jupiter do not seem to exist B\ 



concluded that the length of a day on Mars is a littl< 



longer than our day, or 24 hrs, 39 min. 5 sec. 1 The 



of an accurate measure of the length of day 



in other planets he conceived to be this : " Future 



astronomers may be enabled to make some estimate 



of the general equability of the rotatory motions of 



the planets. Fur if in length of t y should 



perceive some small retardation in the diurnal motion 



occasioned by some resistance of a 

 subtle medium in which the heavenly bodies perhaps 

 move, or, on the other hand, if there should be found 

 an acceleration from some cause or othrr, they might 

 then ascribe the alteration either to the diurnal motion 

 of the earth, or to the gyration of the other planet, 

 according as circumstances, or observed phenomena, 



1 Time of roUtion d.Urmined nnee Hewchel ' dajt, 24 bra. 37 mfc. 

 Bf 



