1889.] of the various Species of Heavenly Bodies. 205 



External Work. 



If external work is done on a comet by meteorites in space, that 

 is to say, if there are collisions with external bodies, the velocity of 

 the comet must be considered in the first place, and the equal or 

 unequal distribution of the masses which it encounters can be tested 

 by the phenomena observed. 



The discussion of the recorded observations shows, indeed, that in 

 addition to the constantly increasing action which takes place in a 

 comet during its approach to perihelion passage, there are at times 

 temporary increases in temperature. 



We know that meteorites are scattered through space, arid here 

 and there are gathered into swarms. It is only to be expected, there- 

 fore, that at times a comet will meet with such swarms just as our 

 own planet does, and in that case its temperature would be increased 

 by the collisions which would occur. The increase of temperature 

 would depend upon (1) the dimensions and density of the swarm ; 

 and (2) upon its velocity. The larger and denser the swarm the more 

 collisions would be likely to occur, and the greater the velocity of the 

 comet the greater the amount of kinetic energy available for transfor- 

 mation into heat energy. 



If the density of the meteoritic plenum increases towards the sun, 

 the external work done will increase with it. 



Collisions between Cometary and other Swarms. 



We have then not only to consider the increased activity in a comet 

 due to its approach to perihelion, but we have also to take into 

 account the possibility of its passing through other swarms of 

 meteorites during its revolution. That such collisions do take place 

 there can be little doubt. Sawerthal's Comet, 1888, which increased 

 in brightness by three magnitudes in two days, is a case in point.* 

 Unfortunately, no spectroscopic observations were made, or no doubt 

 the effects of the increased temperature upon the spectrum would 

 have been apparent. 



The spectroscopic observations of Comet Wells seem to show that 

 this comet also passed through at least one swarm during its revolu- 

 tion. An observation at Greenwich, on May 20th, recorded dark 

 absorption lines, which I have shown to be especial to high tempera- 

 tures in comets. Between that date and perihelion passage (June 

 10th) there were evidences of a lower temperature, as I show in 

 another part of the paper. I am not aware of any observations 

 recording an increase in brilliancy of the comet on May 20th, but if 

 they do exist, they will obviously strengthen this view. 



Perhaps the case of greatest importance, however, is the Great 



* ' Mature,' vol. 38, p. 258. 

 VOL. XLV. P 



