490 Dr. P. Lowell on Photographs of Jupiter 



From the motion of the perijove of the Vth satellite have 

 been deduced:— 



Date. Oblateness. 



Cohn 1896 1 : 15*50 



W. S. Adams 1899 1 : 15-55 



For some reason the oblateness deduced from measures 

 of the orbits of the satellites seems to come out too large. 



Thus La Place got from those of the other satellites 



and De Damoiseau _,„ ., ,. The oblateness has now been so 

 13*49 



well measured that we are certain that these denominators 



are too small. 



We see, then, that the mean of the visual measures and of 



the photographic agree thus : — 



Oblateness, mean of visual determinations * . „ „ L ,„ , 



lo*83 



„ „ photographic determinations t-fto« 



A concordance so striking shows how serviceable these 

 photographs may be for future measures of the rotation 

 periods of different parts of the disk. For the present ones 

 can be compared with those taken at any later date. 



The actual ellipticity of Jupiter combined with its rate of 

 rotation gives us an insight into the planet's present physical 

 condition. 



By Clairaut's theorem the limits between which the ellip- 

 ticity of a rotating body must lie, no matter what the law of 

 its distribution of density be, are \<$> and Jc£ where cj> is the 

 ratio of the centrifugal force to gravity at its equator when 

 the body is homogeneous. Gravity is the attraction there 

 less the action of the centrifugal force, therefore : 



f-rfa Ty_ x 

 a 



where <w = angular rate of rotation, 



T = its period, 

 a = the equatorial radius, 

 g= gravity, 



/ = the attraction of the body on a point at its 

 equator. 



The limit ^>, as the equation by which it is determined 

 shows, corresponds to a distribution of density where all the 



