PROFESSOR TAIT ON IMPACT. 231 



and through the plate and wood a longitudinal slot, 3 inches by § inch, is cut, the centre 

 of the slot coinciding with the centre of the block. Another plate of brass, 3-g- inches by 

 2^ inches, with two parallel slots 2^ inches long and ^ inch broad, half an inch distant 

 from, and on either side of the centre, lies on the fixed plate, and can be clamped to it 

 by means of flat-headed screws passing through the slots. This movable plate has, 

 therefore, a longitudinal (vertical) play of about 2 inches when the screws are loose. 

 It carries the tracing-point and its adjusting mechanism. 



The tracing-point is at the extremity of a steel rod, one inch of whose length is of 

 £ inch diameter, the remaining f inch being of rather less than -§- inch diameter. The 

 thicker part works freely, but not loosely, in a cylindrical barrel, the thinner part passing 

 through a collar at the front end. The cylinder is fixed, at right angles, to the movable 

 brass plate, and passes through the slot in the block. The rod is lightly pressed forwards 

 at the thicker end by a piece of watch-spring, so as to keep it, when required, steadily in 

 contact with the revolving disc. In the wall of the cylindrical barrel is a long slot which 

 runs backwards for \ inch parallel to the axis, and then, turning at right angles to its 

 former direction, runs through a small fraction of the circumference of the barrel. In this 

 slot works a stout wire screwed perpendicularly into the rod which carries the tracing- 

 point. Of course, when this wire is in the transverse part of the slot the needle-point is 

 retracted ; but as soon as it is turned into the axial part the spring makes the needle- 

 point project through the collar. Before the block falls, the wire is in the transverse 

 part of the slot, and the needle-point is retracted. But when, in its fall, the point has 

 passed the edge of the glass disc, a pin fixed at the proper height catches the end of the 

 wire and turns it into the axial slot. As soon as the tracing is complete, the wire is 

 forced back (by means of a system of jointed levers) into the transverse slot, and thus 

 the tracing-point is permanently withdrawn from the disc, so that the block can be 

 pulled up, and adjusted for another fall. 



The last part of the apparatus to be described is that for recording the time. 



It consists of an electrically controlled tuning-fork, making 128 vibrations per second. 

 A circular bar of iron, 8 inches long, is fixed perpendicularly to one of the beams, and 

 in the plane of the beams. From this the tuning-fork is suspended by means of circular 

 bearings. It therefore has a swinging motion perpendicularly to the disc, as well as a 

 translatory motion parallel to it. By means of a screw it can be fixed in any position, 

 and to any degree of stiffness. The bar is at such a height that the end of the tuning- 

 fork carrying the tracing-point is in the same horizontal plane with the centre of the 

 revolving glass plate. By this means it can be adjusted to trace its record anywhere 

 between the edge of the plate and a circle whose radius is 5 or 6 inches, measured from 

 the centre of the glass. 



Theory of the Experiments. 



So far as concerns the motion of the block between two successive impacts, the 

 investigation is extremely simple. For we assume (in fair accordance with the results, 



