1941] WRITINGS OF JOSEPH HENRY, 193 



The mirror is then set so that tlie beam of light is thrown 

 into the room in a line forming the prolongation of the axis 

 of the wheel, which is readily effected by means of a mark 

 previously made on the opposite wall. The beam will pre- 

 serve this direction during the day, since the mirror and 

 the sun revolve with the same velocity, and are therefore 

 comparatively at rest. The only motion of the beam in 

 reference to terrestrial objects is one of rotation on its own 

 axis. If the required direction of the beam is different from 

 that of the first reflection, a second mirror is used. 



Professor Henry's object in exhibiting this article to the 

 Society, was to render this simple contrivance more generally 

 known in our country. He stated that the original inven- 

 tion probably belongs to Dr. Young; that it was at least 

 published by him in 1807, although an account of the same 

 instrument is given in the London Philosophical Magazine 

 for 1883, as a new invention by Mr. Potter. The details 

 of the instrument exhibited differ from those proposed by Mr. 

 Potter, in the addition of a hinge and clamp-screw, by which 

 the axis may be adjusted to the angle of the latitude. The 

 instrument was constructed by an ingenious watch-maker at 

 Princeton ; and its whole cost, including the watch-work, was 

 but sixteen dollars. 



ON THE EFFECTS OF A THUNDER-STORM. 

 (Proceedings of the American Philosophical Society, vol. ii, p. 111-llG.) 



November 5, 1841. 



Professor Henry gave an account of some observations 

 he had made on the effects of a thunder-storm which visited 

 Princeton on the evening of the 14th of July, 1841, 



Storms of this kind (he said) are not very frequent at 

 Princeton; but two severe ones have passed immediately over 

 the place within the last nine years, and the lightning has 

 struck but twice in the village during that time. It is 

 thought by some of the inhabitants that damage by light- 

 ning was more frequent some years ago than it has been 



