NOMOS. 121 



vaporous coat. What then, we may ask, are the 

 properties of this coat? This is the question. Is it 

 capable of reacting more vigorously with the Sun 

 than the coat which has been vaporised ? Does it 

 hold the same relation to this outer coat that land 

 does to water ? The assumption is neither impossible 

 nor improbable, for it may be assumed that this inner 

 coat is less rare than the outer coat because it is 

 nearer the centre of the comet, and because it is less 

 rare (inasmuch as the power of reacting is in some 

 sense directly proportional to density) greater react- 

 ing powers may be ascribed to it. And if so, then 

 we may understand how the comet may begin to 

 move outwardly from its perihelion, and that this move- 

 ment may continue for some time with great and 

 scarcely diminishing velocity ; for if the increasing 

 resistance in space, as the comet approached the Sun, 

 was sufficient for some time to neutralise the incre- 

 ment of force arising from the diminishing distance 

 between the Sun and comet, the diminishing resistance 

 in space as the comet moves away from the sun may 

 for a long time conceal the loss of force arising from 

 the increasing distance between the Sun and comet, 

 the smaller amount of force serving to propel the 

 comet to the same distance under the diminished 

 resistance. We may, indeed, without any difficulty, 

 imagine all this to go on until the comet is restored 

 to its original position in the orbit and to its original 

 condition, for as it leaves the Sun it resumes the coat- 

 ing which had been vaporised by the heat of the solar 

 rays. And thus, the great eccentricity in the orbits 

 of comets, instead of being a matter of difficulty, 



