THE TWO COMETS OF THE YEAR 1868. 167 



sphere's density, is most effective in the sun's neighbour- 

 hood. We know, for instance, that a balloon must not 

 be fully inflated at first rising, because when- it reaches 

 the upper regions of air, where there is less compres- 

 sion, the enclosed gas expands and would burst the 

 silk if the balloon had been fully filled at first. And 

 certainly, on the somewhat bold assumption that the 

 solar atmosphere extends outwards to those regions in 

 which the observed change of volume takes place, and 

 on the additional and equally bold supposition that 

 comets are surrounded with a film of some sort per- 

 forming the same function as the silk of the balloon 

 (or that in some other way the substance of the comet 

 is prevented from intermingling with the substance of 

 the solar atmosphere) the theory of M. Valz would 

 have a certain air of probability. Even then, however, 

 it would be insufficient to account for the enormous 

 extent to which the variation has been observed to 

 proceed. 



The only probable explanation of the variation in 

 question is that which is put forward by Sir John 

 Herschel in his admirable work on the southern 

 heavens. During his stay at the Cape of (rood Hope 

 he had an opportunity of observing the recession of 

 Halley's comet, and he discusses the phenomena with 

 admirable acumen and judgment. The result at which 

 he arrives appears to afford a simple and rational ex- 

 planation of the observed phenomena. He supposes 

 that as a comet approaches the sun the action of the 

 solar heat transforms the nebulous substance of the 



