4b'0 Mr. A. S. Davis an a possible Cause of 



whole surface might in turn become exposed to the sun's rajs, 

 and evaporation would probably take place even on the side 

 turned away from the sun. But unless the body be of a re- 

 gular shape, the effect of evaporation will be to gradually stop 

 any rotation which it might at first have ; for the force of 

 recoil from the evaporation would act upon it in the same way 

 as the wind does on a vane, and it would at length take up a 

 position with its longest axis in the direction of the sun. 



Now let V be the average velocity relative to the body with 

 which the molecules escape from it. Let M be the mass of 

 the body just before the escape of a molecule of mass /jl from 

 it. Then the velocity due to the recoil as this molecule escapes 



will be — V. Now the molecules as they evaporate will start 



off in various directions; but almost always more or less towards 

 the sun. Let us suppose, as being not far from the truth, that 

 the inclination which their directions have to a straight line 

 passing through the sun is on the average 45° ; then the ave- 

 rage velocity due to recoil acquired by the body on the evapo- 

 ration of mass dm will be — • -—=, and the velocity acquired 



m s/ 2 ./i 



whilst the mass of the body is being reduced by evaporation 

 from jn-L to m 2 will be 



V X 1*63 X login — • 



( m idm _ 

 m 



V '2J H22 m - * * w m 2 



Now the average velocity of hydrogen molecules at 0° C. is 

 1*06, of oxygen *266, and of water vapour *35 mile per second. 

 For the sake of illustration, let us suppose that V = -35, and 



7 —± = 1000000, these being the values we should have to assign 



if the body were a block of ice containing one gramme of sand 

 or any other non-volatile substance, the block itself being equal 

 in mass to a cubic metre of water. The velocity due to recoil 

 by evaporation would then be 3*42 miles per second, or about 

 295,000 miles per day. A tail would thus be formed which 

 would increase in length nearly a million miles in every three 

 days. The visible portion of this tail would consist of solid or 

 liquid matter which had resisted evaporation; but there would 

 also be present in the tail a large portion of the gas formed 

 during evaporation ; for since the evaporating gas has a velo- 

 city relative to the body from which it is evaporating of *35 

 mile per second, those portions of the gas which have evapo- 

 rated since the body acquired by recoil a velocity greater than 

 •35 mile per second will be also carried backwards into the 

 tail. The estimate of the rapidity of tail- formation 1 have 



