Mr. Dana's Reply to Mr. Couthouy's Vindication. 129 



and it follows from the principle just established, that unless the 

 velocities of emission augment as rapidly as the velocity of revo- 

 lution, the deviation in question will increase to the perihelion 

 and afterwards decrease ; as it is in fact known to do. It ap- 

 pears therefore that the theory explains ■ the general phenome- 

 non of the situation of the tail. 



We see also, in the light of this theory, why it is that the neb- 

 ulosity is confined to the side of the nucleus that is turned to- 

 wards the sun. All portions of nebulous matter that may at any 

 time chance to be on the other side are soon expelled. As the 

 sun does not act upon the farther side of the nucleus, there is 

 probably little if any matter ever rising from that side. 



The comparative indistinctness, sometimes noticed, of the fol- 

 lowing side of the tail may arise from the fact that the sun has 

 acted during a shorter period of time upon that side of the nu- 

 cleus. 



I will only remark farther, that it is no objection to the theory 

 which I have been advocating, that, if it be true, comets must be 

 wasting away by reason of the continual escape of the matter of 

 which they are composed, during each period of their approach 

 to the sun. For observation has long since led astronomers to 

 believe that such a waste is in actual progress. In fact there is 

 nothing to forbid our supposing that the faint telescopic comet 

 which steals through our firmament, almost unobserved, was 

 once a very prince in the skies, and gloried in as long a train as 

 did our illustrious celestial visitor of 1843. 



Art. XV. — Reply to Mr. Couthouy's Vindication against the 

 charge of Plagiarism ; by James D. Dana, Geologist of the 

 United States Exploring Expedition. 



We cannot but regret that it has become necessary to intrude 

 so unpleasant a controversy, as that before us, on public attention, 

 and I should take my part in it with extreme reluctance, were it 

 not urgently required, as well by the interests of science as of 

 truth. If possible, I would now gladly drop the subject, espe- 

 cially because of its bearing upon Mr. Couthouy's character : but 

 this may not be. No ill will towards Mr. Couthouy instigated 

 the charge at Albany, but solely a regard for right j much of the 



Vol. xlvi, No. 1.— Oct.-Dec. 1843. 17 



