36 ON THE CONSTRUCTIOX ANP USE OF 



A ^lass rod (/, a\ Fig. 33, is sustained by two wheels />. h' , and kept in contact 

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

 carries a circnlar frame (7, </', upon which at e,i',e", are three gla.ss holders and 

 platinum catches. A spring / holds the sensitive plate in ])osition, by pressing 

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

 roller 7, which runs on a glass rod //, 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 tlie glass rod a. From m to a fixed point near h, a stri)) of elastic India- 

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

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

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

 brouo-ht into coincidence with the apparent path of the moon. At o is a tlauul)- 

 screw or clamp Through the ring rt, 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. 



Tlie 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. 1 can follow the moon for nearly four minutes, but liave never retpured 

 to do so for more than fifty seconds. .\t the montli of the instrument, where no 

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



The setting of the frictionless slide in angular position is accom])lished as follows : 

 A groimd glass ])late is put into it, with the ground face toward the mirror. ITpon 

 this face a black line must have been traced, precisely parallel to the ^d n. 'I'liis 

 mav be accomplished by firmly fixing a pencil point against the grotuid side, and 

 then (h-awing tlie frame d and glass past it, while the rest of the slide is held fast. 

 As tlie moon passes across tlie field, the ])osition of the apparatus must be changed, 

 rmtil one of the craters runs along the line from end to t'lid. A cross line ilrawn 

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

 see, and A\hen a crater is kept steadily on tlie 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 sensiti^'e 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. The 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 Avith vmiformity. In addition, by these 

 means an unlimited power could be brouglit 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 colunm shortened, by a conical vessel ; but it was soon perceived that 



