98 



five inches in diameter, which is so geared into the large wheel of 

 the watch as to make one turn in twenty-four hours. The axis of 

 this wheel is a steel rod, carrying on its upper end a small mirror, 

 which can be set in any position by means of an universal joint. The 

 watch work and the wheel are attached to the mahogany block by a 

 hinge, so that the axis of the wheel can be inclined to the horizon at 

 an angle precisely equal to the latitude of the place where the instru- 

 ment is to be used. 



The adjustment of the instrument is very simple. It is placed on 

 the outside of the window, with the axis of the wheel parallel to the 

 axis of the earth; a meridian line having been traced on the window 

 sill for this purpose. The mirror is then set so that the 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 preserve 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 invention 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 and Philoso- 

 phical Magazine for 1833, 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 watchmaker at Princeton ; and its whole 

 cost, including the watch work, was but sixteen dollars. 



Dr. Patterson and Mr. Walker read strictures by Mr. Hass- 

 Ier upon a notice of the Massachusetts State Survey which ap- 

 peared in the Proceedings for June last, and commented on the 

 apparent misconceptions of Mr. Hassler, as did several other 

 gentlemen. 



Professor Bache communicated, on behalf of Professor Rum- 

 ker, of Hamburg, the observations made in 1838, at the obser- 

 vatory in that city, on Encke's comet, with their reductions. 



