108 On the Formation of the Tails of Comets. 



Mode of Formation of the Tails of Comets. 



To proceed with due caution, in approaching the field of inqui- 

 ry before us, we must seek for some solid ground to advance up- 

 on. This may be found in the following statements. 1. The 

 general situation of the tail of a comet with respect to the sun, 

 shows that the sun is concerned, directly or indirectly, in its for- 

 mation. We have seen too, that the changes which take place 

 in the dimensions of a comet, both in approaching the sun and 

 receding from him, conduct to the same inference. 2. Since the 

 tail lies in the direction of the radius-vector prolonged beyond the 

 head, the particles of matter of which it is made up, must have 

 been driven off from the head by some force exerted in a direc- 

 tion from the sun. 3. This force cannot emanate from the nu- 

 cleus, for such a force would expel the nebulous matter surround- 

 ing the nucleus in all directions, instead of one direction only. 

 This objection, it is true, would be obviated, if a repulsive action 

 were to be exerted by the nucleus only, from that hemisphere 

 which is on the side opposite to the sun ; but it will be at once 

 perceived that such a force as this would not give the tail the 

 form and direction which it is known to have, when it is consid- 

 ered that the tail has its origin in the nebulosity at the side of the 

 nucleus. A repulsion from the nebulosity is also out of the ques- 

 tion, as, on this supposition, the matter would be expelled out- 

 wards in all directions. In fact no one force having its origin in 

 the head of the comet, can be conceived of. that would be ade- 

 quate to the production of the tail. A tangential force at first 

 view, might seem to be, but this would only form a tail proceeding 

 from one side of the nucleus. The idea of two or more tangen- 

 tial forces can hardly be entertained, as, besides its inherent im- 

 probability, it involves the idea of matter in different states, or of 

 different kinds in different parts of the tail, for which we have 

 not the least shadow of evidence. It is possible however, as we 

 shall see farther on, that a repulsive action of the nucleus upon 

 the matter of the nebulosity, may be combined with some other 

 force foreign to the comet, as an auxiliary cause in producing the 

 phenomena of the tail — its effects on the side towards the sun 

 being contracted, at a certain distance/rom the nucleus, by this 

 latter force.* 4. There seems, then, to be little room to doubt 



* Although, as I have asserted in the context, there is no one force immediately 

 related to the nucleus that can form the tail, it is perhaps possible to conceive of 



