lO 



T. C. CHAMBERLIN 



well know, represents a stage at which a notable atmosphere and a 

 distinctly massive hydrosphere have been acquired and held. The 

 four bodies thus represent those stages of evolution which are most 

 significant in such a study as this. They are all notably dense 

 compared with the great planets that lie outside them and with the 

 sun at the center of the system. The moon, Mars, and Venus will 

 be treated as representing a t5q3ical series of stages of evolution 

 connecting the small atmosphereless type with the largest known 

 cold planet enveloped with a deep water-sphere and gas-sphere, 

 the earth. No special study will be given to the large hot bodies 

 of low density that form the great outer group.^ 



STATISTICAL DATA 



Some of the more essential statistics on which the study will 

 be based are gathered into Table I. 



TABLE I* 



* These statistics are taken from Moulton's Introduction to Astronomy, Revised Edition, 1916. Venus 

 has no satellite and its mass and density can only be determined by indirect means which are not very 

 accurate, and hence the figures for these are marked with an interrogation point, but they are probably 

 close enough for our purpose. The figure 5.53 for the density of the earth is conservative; figures as 

 high as 5.56 and 5.57 have been used. These higher figures would give greater shrinkage. The dimen- 

 sional data are given in miles and in kilometers, but the computations are carried out in miles, because, 

 being the larger unit, it is the more convenient. An even larger unit is desirable for most earth studies 

 and so the standard degree of a great circle of the earth is added in circumferential measurements. Degrees 

 are convenient units in working with globes. 



THE METHOD OF THE INQUIRY 



As a step preparatory to the proposed comparison there were 

 built up from the moon, Mars, and Venus, each in turn, by using 

 material of its own mean density, parity-earths whose masses were 

 equal in each case to that of the actual earth. A similar parity- 

 earth was built up of mean meteoritic material. The radii and 

 volumes of these parity-earths were then computed and taken as 



' Cf. W. D. MacMillan, "On Stellar Evolution," Astrophys. Jour., Vol. XL VIII, 

 No. I (July, 1918), pp. 40-41. 



