428 Sir J. F. W. Herschel on a Machine 



must be turned truly cylindric, leaving at either side a slight ele- 

 vation like a parapet to prevent the thread from slipping off". In 

 the front edge of this elevation must be cut a small notch F, 

 in which the end of the thread is to be carried off the edge of 

 the wheel, and fastened on a pin P stuck (like the tuning pegs 

 in the handle of a violin) into the face of the wheel, so as to allow 

 of lengthening or shortening the thread a little by winding it 

 more or less on this peg. The other end of the thread where it 

 leaves the circumference of the wheel at E hangs down vertically, 

 and has suspended to it a vertical, straight, divided scale MN, the 

 divisions of which are marked and read off by the intersection of 

 its fiducial line MKN with the horizontal straight edge IKJ at K, 

 for which reading off a microscope moveable on IJ, and furnished 

 with a micrometer, might be used should such a degree of pre- 

 cision be required. The straight edge I J is itself also graduated 

 for a purpose to be presently explained, and might in like manner 

 be read oft* microscopically. 



(5.) The axis C also carries on it a barrel a be, round which 

 and round a fixed smooth pin x a string makes two or three 

 coils (embracing both the barrel and pin in each coil), after which 

 it is attached at both its ends to a weight w, which must be such 

 as to afford a sufficient tension to the string to produce a smooth 

 uniform friction on the barrel, and thereby to prevent the axis 

 from turning without the continual application of a moving power, 

 and to bring it at once to rest, without jerks, dragging, or recoil, 

 when the power ceases to act. The power may be applied either 

 at the circumference of the barrel by the hand, or by a handle 

 fixed on the axis C, and not represented in the figure. The barrel, 

 weight, and pin x, are supposed to lie behind the plane of the 

 index circle AH, which is that of the paper— the rest of the ap- 

 paratus before it. 



