36 ON THE CONSTRUCTION AND USE OF 



A glass rod a, a', Fig. 33, is sustained by two wheels b, 5', and kept in contaci, 

 with them by a third friction roller c, pressed downward by a spring. This rod 

 carries a circular frame d, d', upon which at e,c',e", are three glass holders and 

 platinum catches. A spring / holds the sensitive plate in position, by pressing 

 against its back. The circular frame d is kept in one plane by a fourth friction 

 roller </, which runs on a glass rod h, and is kept against it by the inward pressure 

 of the overhanging frame d. The cord i is attached to the arm &, and pulls in the 

 direction of the glass rod a. From in to a fixed point near 6, a strip of elastic India- 

 rubber is stretched, to keep the cord tight. The ring of brass n, n' carries the 

 whole, serving as a basis for the stationary parts, and in its turn being fastened to 

 the eyepiece holder, so as to allow the glass rod a to change direction, and be 

 brought into coincidence with the apparent path of the moon. At o is a thumb- 

 screw or clamp Through the ring n, n', a groove^ is cut, into which a piece of 

 yellow glass may be placed, when the actinic rays are to be shut off from the plate. 



Since this contrivance has been completed, all the previous difficulties have 

 vanished. The moving of a plate can be accomplished with such precision, that 

 when the atmosphere was steady, negatives were taken which have been enlarged 

 to three feet in diameter. 



The length of time that such a slide can be made to run is indefinite, depending 

 in my case on the size of the diagonal flat mirror, and aperture of the eyepiece 

 holder. I can follow the moon for nearly four minutes, but have never required 

 to do so for more than fifty seconds. At the mouth of the instrument, where no 

 secondary mirror is necessary, the time of running could be increased. 



The setting of the frictionless slide in angular position is accomplished as follows : 

 A ground glass plate is put into it, with the ground face toward the mirror. Upon 

 this face a black line must have been traced, precisely parallel to the rfod a. This 

 may be accomplished by firmly fixing a pencil point against the ground side, and 

 then drawing the frame d and glass past it, while the rest of the slide is held fast. 

 As the moon passes across the field, the position of the apparatus must be changed, 

 until one of the craters runs along the line from end to end. A cross line drawn 

 perpendicular to the other, serves to adjust the rate of the clepsydra as we shall 

 see, and when a crater is kept steadily on the intersection for twice or three times 

 the time demanded to secure an impression, the adjustment may be regarded as 

 complete. 



It is necessary of course to expose -the sensitive plate soon after, or the apparent 

 path of the moon will have changed direction, unless indeed the slide is set to suit 

 a future moment. 



b. TJie Clepsydra. 



My prime mover was a weight supported by a column of sand, which, when the 

 sand was allowed to run out through a variable orifice below, could be made to 

 descend with any desired velocity and yet with uniformity. In addition, by these 

 means an unlimited power could be brought to bear, depending on the size of the 

 weight. Previously it was proposed to use water, and compensate for the decrease 

 in flow, as the column shortened, by a conical vessel ; but it was soon perceived that 



