1910] 071 Hallei/s Comet. 763 



image was detected on a plate taken at Helwan in Egypt on August 

 24, with the Reynolds reflector, the mirror of which was made by 

 the late Dr. Common, F.R.S. 



What of future returns ? Can we expect reappearances of the 

 comet to continue indefinitely ? Our knowledge of the nature and 

 history of comets, though it has advanced rapidly in the last feAV 

 decades, is still scarcely sufficient to enable us to answer with confi- 

 dence ; but the indications are that comets are continuously being 

 disintegrated and are ultimately broken up, perhaps into a swarm of 

 meteors. The tail of a comet probably represents its losses at the 

 moment. The tail or train does not, as migiit be supposed, follow 

 behind the head in the same path, as the smoke follows an engine : it 

 is as often in front of the head as behind it. The tail always, in fact, 

 points away from the sun, as though a strong current of air were 

 blowing in all directions outwards from the sun, determining the 

 direction of the tail as the wind determines that of a streamer. And 

 there is actually this in common between the cause of the tail and 

 a current of air — that botii have a tendency to drive away lighter 

 particles from heavier. We blow away chaff from grain : and the 

 fierce Uglit-pressure of the sun (to which many astronomers now attri- 

 l)ute the formation of cometary tails) in the same way separates the 

 lighter constituents of tlie comet and drives them outward into space. 

 Possibly we are wrong in assigning such large powers to light pressure 

 — the older view that the repulsive action is electrical may turn out 

 to be more correct — but that will not alter the nature of the separat- 

 ing action, which depends on the fact that the repulsion varies as the 

 surface of a particle, and therefore as the square only of its linear 

 dimensions, while its mass varies as the cube. By reducing the 

 dimensions we thus give the repulsion greater relative importance ; 

 halve the size of a particle and it is twice as easy to blow away ; halve 

 it again, and the facility is again doubled, and so on ; and this is true 

 w^hether we are concerned with light-pressure or electrical action, or 

 the blowing of dust. 



Comets thus tend to grow smaller. The losses represented by the 

 tail are difficult to estimate quantitatively : but from recent photo- 

 graphs, especially those of Prof. E. E. Barnard, and those taken at 

 the Royal Observatory, Greenwich, it has been conclusively proved 

 that matter is travelling outwards from the head of the comet, and 

 some progress has been made with quantitative estimations. Mr. A. S. 

 Eddington recently gave an interesting account of such work in this 

 Institution (Friday, March 26, 1909). 



The hypothesis is not by any means new. " On this hypothesis, 

 said Dr. Huggins in 1882, "a comet would of course suffer a large 

 waste of material at each return to perihelion, as the nucleus Avould be 

 unable to gather up again to itself the scattered matter of the tail : 

 and this view is in accordance with the fact that no comet of short 

 period has a tail of any considerable magnitude." But in recent years 



