ON THE INFLUENCE OF COMETS. 229 



that this phenomenon is of a nature to occur frequently, and may 

 possibly produce those epidemic diseases which have been attributed 

 to it.* Thus, also, Mr. Herapath has considered the subject : 

 '* history furnishes us with numberless instances of the great atmos- 

 pheric changes which have accompanied or succeeded the apparition 

 of large and notorious comets; and unless we reject altogether its 

 often iterated testimonies, I do not see how we can refuse our assent 

 to influences so manifest, and yet so simple and perfectly philosophi- 

 cal. That comets may produce very sensible effects, in the way 

 alluded to, not only in the meteorological state of the air, but in its 

 salubrity, our best philosophers admit ; but these effects must always 

 be concurrent or posterior — ^never, I apprehend^ anterior — to their 

 appearance."t 



Without offering any opinion upon the views thurs suggested, 

 which, it must be confessed, have been lately placed in a very unfa- 

 vourable light by Professor Littrow, I may remark that this is a 

 most important and interesting portion of the subject, and totally 

 distinct from that which has formed the obje-ct of our reflections: 

 the influence, whose possibility we have been contemplating, would 

 be as independent of proximity or contact, as that of light or gravi- 

 tation ; but that indicated by M. Arago is an action resulting from' 

 the real combination of a heterogeneous substance with our atmos- 

 phere. That such an union may occur, is indisputable, from the 

 disposition of the paths of comets, the immense extent of their 

 tails, and their feeble gravitation towards their nuclei ; and it is 

 highly probahle that events of this kind have actually taken place : 

 but whether any effects are thus produced, and what those effects 

 may be, are questions which cannot be determined otherwise than 

 by observation and experience. 



THOMAS WILLIAM WEBB. 



Tretire, near Ross, Dec, 3, 1835. 



* Mechanics' Magazine, xviii, 28, 60. 

 f Mechanics' Magaziney xviii. 63. 



