122 On the Figure of the Planet Mars. 



greatest value which e can have is when e=2,; and in this case 



5?D hg 



^■2(2D-3)-6(D / -l)- 10 _ 6 D_; 



In the case of the Earth, the values most commonly admitted 

 for the mean density of the planet and its solid crust give in 

 round numbers D = 5*6 and D,=2*6. With these numbers it 



readily appears that e cannot exceed -pr^* 



The smallest value that could be given in the present state 

 of our knowledge to D would make it nearly equal to twice D 7 , 

 and therefore 



_5 1 



e ~ 7 ? "404-6* 



Thus, as I have already pointed out, the abrasion theory 

 cannot account for the Earth's figure as perfectly as the theory 

 of entire original fluidity. If Mars were a homogeneous solid, 

 the abrasion theory would as well account for the observed 

 ellipticity as for a homogeneous fluid; for in both cases e would 

 then be 



5 , 1 



4? ° re = 179^4' 



a value which is sensibly larger than the result of the best 

 observations. 



The researches of various astronomers have recently shown 

 that the surface of Mars presents a well-defined distribution of 

 land and water. The land seems to be grouped in islands, and 

 not in great continents. Now, if the figure of the planet de- 

 viated from that deduced on the hypothesis of original fluidity, 

 if its oblateness were much less or much greater, such a distri- 

 bution of land and water could not exist. With a large ob- 

 lateness the land would be arranged in a great belt about the 

 equator ; and with a small oblateness or a spherical figure the 

 land would form two circumpolar continents with an interme- 

 diate equatorial ocean. All recent observers concur in an en- 

 tirely different distribution from either of these ; and the results 

 of physical theory, measurement of diameters, and graphical 

 representations of the planet's surface seem to be in harmony, 

 and the difficulty which was formerly supposed to exist in 

 accounting for the figure of Mars may be now considered as 

 completely removed. 



