Oi? ARTS AND SCIENCES. 185 



XI. 



UNDULATIONS OBSERVED IN THE TAIL OF COGGIA'S 

 COMET, 1874. 



By L. T r o u V e l o t. 



Presented Nov. 14, 1877. 



On the evening of July 21, 1874, at D*" 0™, the moon being in her 

 first quarter, and the sky remarkably clear even close to the horizon, 

 my attention was attracted by a bright ray of light darting from 

 the north-western horizon, way up in the constellations. Taking it 

 for an auroral phenomenon, I went in for the spectroscope ; but on my 

 return, after a few seconds, to my disappointment I found no more 

 trace of it. Soon, however, it reappeared, and darted up in an instant 

 after the manner of certain auroral rays, and vanished again after ten 

 or fifteen seconds. I then became aware of my error, and found with 

 surprise that the phenomenon was taking place in the tail of Coggia'a 

 comet, the head of which was then plunged under the horizon. 



During the whole time that I observed this interesting phenomenon, 

 I saw the comet's tail shortening and extending, lightening up and 

 extinguishing like the rays of certain auroras. Extended undulations, 

 rapid vibrations, ran along it in succession from the horizon to its 

 extremity, giving it the appearance of a fine gauze wavering in a 

 strong breeze. The pulsations and the waves of light were of unequal 

 duration ; some being rapid, while others lasted a longer time. For 

 over one hour, the comet's tail kindled and extinguished more than 

 one hundred times ; the extinction being sometimes so complete that 

 it was impossible to see any trace of the comet ; while sometimes it 

 became so bright that, in spite of the light of the moon, it could be 

 distinguished easily in all its contours, even to its very extremity, 

 which was then a little to the south of y Ursa Majoris. 



Be it coincidence or not, at the moment that this phenomenon was 

 occurring, a strong magnetic perturbation was going on in Cambridge, 

 where the declination needle oscillated through an angle of I'' 27', 



