194 WRITINGS OF JOSEPH HENRY. [18*1 



lately; and the idea has been suggested that the water of the 

 canal, which passes to the south of the place, may have had 

 some effect in determining the course of the cloud. Be this 

 as it may, the thunder-storm generally comes from the south- 

 west, and before it reaches the village it usually divides into 

 two parts, one of which passes along the edge of the Rocky 

 Hill, and the other along the valley of Stonybrook, so that the 

 principal part of the storm seldom passes immediately over 

 the village ; and when it does thus pass it is generally at a 

 great elevation, and the thunder is not so loud as that which 

 the observer has been in the habit of hearing at the north. 

 In connection with this remark. Professor Henry mentioned 

 that he has several times observed the lightning assume a 

 beautiful violet color, similar to that of the vapor of iodine, 

 and this was particularly the case during a storm which 

 occurred during the 12th of April, 1840. On this occasion, 

 although the cloud and the flashes appeared directly over- 

 head, yet the sound of the thunder seemed to come from a 

 distance. The peculiar color may perhaps receive a suflfi- 

 cient explanation by referring it to the fact of the discharge 

 taking place at a great altitude, and consequently in com- 

 paratively rarified air, as in the case of the color exhibited 

 by the spark through a vessel partially exhausted. 



The storm of the evening of the 14th of July, was said to 

 be more severe than any which had visited Princeton for 

 twenty years before. It commenced between 7 and 8 o'clock, 

 and lasted about three hours: the thunder was almost con- 

 tinuous, but except in two or three cases it was not ver}' near. 

 Several buildings and other objects were struck in the 

 vicinity of Princeton ; and also Mrs. Hamilton's house, which 

 is situated in the village, about twenty rods west of the col- 

 lege, on the opposite side of the way. It seemed a little 

 surprising that this house should be singled out, since the 

 buildings on either side are considerably higher, although 

 at a few rods distance, and in front of the one to the west is 

 a number of tall trees. The house is also furnished with a 

 lightning rod ; but this, like most of the rods erected in the 

 country, is not formed in accordance with the most scientific 



