My brother, Mr. Daniel Draper, to whose mechanical ingenuity I have on several occasions been indebted for assistance in the manifold difficulties that have arisen while constructing this telescope, continued these experiments at intervals. He presented me on my return with a slide and sand-clock, with which some excellent photographs have been taken. He had found that unless the slide above mentioned was made ungovernably long, the same trouble continued. He then ceased catching the sliding frame _h_, Fig. 32, by two opposite sides, and made it run along a single steel rod _a_, being attached by means of two perforated plates of brass _b_, _b′_. The cord _i_ going to the sand-clock, was applied so as to pull as nearly as possible in the direction of the rod. A piece of cork _c_, gave the whole steadiness, and yet softness of motion. The lower end of the frame was prevented from swinging back and forward by a steel pin _d_, which played along the glass rod _e_. All these parts were attached to a frame _k_, fitting on the eyepiece holder, and permitting the rod _a_ to change from the horizontal position in which it is here drawn, to any angular one desired. The thumb-screw _f_ retained it in place; _g_ and _g′_ are pulleys which permit the cord to change direction. [Illustration: Fig. 32. Sliding Plate-holder.] Subsequently, a better method of examining the uniformity of the rate, than by noticing the sharpness of the photograph produced, was invented. It consists in arranging a fixed microscope, magnifying about 40 times, at the back of the ground glass plate, which fits in the same slide as the sensitive plate. By watching the granulated appearance pass before the eye, as the slide is moved by the clock, the slightest variation from uniformity, any pulsatile or jerking movement is rendered visible. By the aid of this microscopic exaggeration, it was seen that occasionally, when there had been considerable changes in temperature, the steadiness of the motion varied. This was traced to the irregular slipping of _b_, _b′_. [Illustration: Fig. 33. Frictionless Slide (front view). Sectional view.] A different arrangement was then adopted, by which a lunar crater can be kept bisected as long as is necessary, and which gives origin to no irregularities, but pursues a steady course. The principle is, not to allow a slipping friction anywhere, but to substitute rolling friction, upon wheels turning on points at the ends of their axles. The following wood-cut is half the real size of this arrangement.