8 



Mr. J. H. Poynting on a metliod of [Nov. 21, 



longed out of the case to the observer, these two sliding pieces can be 

 made to approach, and clamp the rod, or to recede and free it. By 

 having the opposite surfaces of the sliding pieces and the rod polished 

 and clean, it is possible to clamp and unclamp without producing any 

 disturbance. The clamp is of great use also to lessen the vibrations when 

 they are too large, as it may be brought into action at any moment, and 

 on releasing carefully the beam will start again from rest without any 

 impetus. It may be used ioo to increase the vibrations by releasing 

 suddenly when the beam will have a slight impetus in one direction 

 or the other. 



The weights which I have compared are two brass pounds avoirdu- 

 pois, made for me by Mr. Oertling, and marked A and B respectively. 

 They are of the usual cylindrical shape with a knob at the top 

 (fig. Ill, 2) . Two small brass pans (fig. Ill, 3) with a wire arch by which 

 they can be suspended, are used to carry them ; these are called 

 respectively Xand T. I found on beginning to use them that there was 

 too great a difference between A and B. I therefore adjusted them 

 by putting a very small piece of wax upon A, the lighter. But the 

 difference between them increased by 0"0782 mgm. in two days, which 

 I thought was probably due to the wax. After the fourth series I 

 therefore removed it and scraped B till it was more nearly equal to A. 

 The weighings I — IV have, however, been retained, for though the 

 differences on different days vary they are fairly constant on the same 

 day. 



The weights are changed by the following apparatus which has been 

 designed to effect the change as simply and quickly as possible. 



A horizontal " side rod " or link (ss, fig. V) is worked by two 

 cranks (cc, fig. V, 2), which are attached to the axles of two equal 

 toothed wheels (tt) with a pinion (p) connecting them. A second 

 pinion (q), on a rod prolonged out of the case to the observer, gears 

 with one of the toothed wheels. By turning this rod the toothed 

 wheels are set in motion, both in the same direction, moving the 

 horizontal " side rod " from the right say upwards and over to the 

 left. A pin (pn) stops its motion downwards further than is shown 

 in fig. V, 1. Near each end of the rod is cut a notch, and across these 

 are hung the pans carrying the weights. The apparatus is fastened to 

 the floor of the case between the central upright supporting the beam 

 and the scale pan, the side rod being perpendicular to the direction of 

 the beam, and exactly over the centre of the pan, In fig. Y, 1, one of 

 the weights B is supposed to be resting on the scale pan (the wires 

 suspending the pan from the beam not being shown), the side rod 

 having moved down so far below the wire of the smaller pan carrying 

 the weight that it leaves it quite free. If, now, it is desired to change 

 the weights the rod R is turned, setting the wheels in motion, the side 

 rod moves up, picks up B — the notch catching the wire — then travels 



