73 



There does not, therefore, seem to be any bound which can 

 be set as the outer limit of the earth's atmosphere of this class, 

 except the limit of the earth's control. This limit of control 

 on a line between the earth and the sun is, accordng to Moul- 

 ton, 620,000 miles from the earth's center, and in the opposite 

 direction, 930,000. These are the minimum and maximum 

 radii of the spheroidal space within which the earth exercises 

 greater differential attraction on molecules than the sun. This 

 then may be taken as the strictest limitation of the earth's atmos- 

 phere. 



Between the coUisional atmosphere and this outer limit, the 

 molecules in shooting out and returning are liable to collide with 

 one another, and the rebounds from such encounters are liable 

 to be in any direction. It therefore follows as a necessary 

 consequence that if such a collision takes place when the ve- 

 locities are of the higher order, and if the rebounds are more 

 or less tangential to the earth's surface, one or both of the col- 

 liding molecules may pass into orbits about the earth, having 

 such form that they will not encounter the collisional atmosphere. 

 Inspection shows that of the molecules that have fallen back 

 toward the earth a distance greater than its radius and which 

 encounter molecules going outwards with like velocities, a no- 

 table percentage will pass into orbits instead of returning to the 

 earth. This leads to the conception of an atmosphere formed 

 of molecules that behave as satellites of the earth and are di- 

 vorced, for the time being, from the collisional system and from 

 the fountain-like system. It is important to observe further 

 that this divorce lasts as long as the revolutional molecule es- 

 capes further encounter, and so, if we assume that the number 

 of these is small, we at the same time impliedly assume that they 

 have an excellent chance of remaining for long periods. This 

 implies that the accumulation of such molecules may go on for 



