472 PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



to prevent shake and loaded with a fixed cylinder. These springs keep the 

 horizontal screws always in tension. 



When the brackets B, figs. 4 and 5, are near their highest positions, the circular 

 top-plates P'P' of the slide rests project through holes in the phosphor-bronze plate, 

 PP, fig. 5, which forms the base of the balance. Copper spinnings s of section ~\ fit 

 closely round the top-plates, and can slide between the plate P and ring r, thus 

 forming a draught-tight joint, and at the same time permitting horizontal motion of 

 about half-an-inch in any direction. 



For supporting the marble cylinders M, fig. 5, annular phosphor-bronze castings C, 

 of inverted channel section, rest on fine-threaded levelling screws I, projecting through 

 the top-plates of the slide rests, the heads being below the plates, so that levelling 

 can be done from beneath the balance case. This arrangement is on the " hole, slot 

 and plane principle," to avoid constraint and yet ensure precision in position. 



SECTION 4. THE PHYSICAL BALANCE. 



A photograph of the instrument, without coils, is shown in fig. 6, Plate 8. It 

 has a beam 20 inches (50'8 centims.) long, capable of supporting 5 kilogrammes at 

 each end, and turning with about one-tenth of a milligramme, a rider beam, divided 

 into 100 parts on each side, and two rider carriers are fitted. All the knife edges 

 and planes are of agate, and as fine as possible consistent with the loads they have 

 to carry. 



From each of the outer knife edges K there depends a three-armed spider S, with 

 heavy nuts N at the end of each arm, and adjustable hooks a, a, from which the 

 corresponding suspended cylinder hangs on three phosphor-bronze wires w, w, w. 

 The object of the nuts is to enable the suspended cylinder to be levelled, two very 

 sensitive levels being fixed to the cylinder for this purpose. 



Below the suspended cylinder, and quite clear of it, is a copper disc d, fig. 6, 

 carried by three wires w', w', w' attached to the clamping beam F of the balance, for 

 supporting the cylinder should one of the wires w, w, w become unhooked. 



The scale pans for carrying the weights used to balance the forces exerted by the 

 coils, hang from separate planes on the same knife edges as support the suspended 

 cylinders. These may be seen in fig. 7, where K is the knife edge, H is the hook 

 carrying the spider S, and h the hook supporting the scale pan p. This arrangement 

 is novel, and of considerable utility, for it permits of removal or replacement of the 

 weights without affecting the levelling of the suspended cylinder. Its adoption, 

 however, necessitates the perfect straightness of the knife edges. This condition has 

 been satisfied to a very high degree of accuracy by Mr. OERTLING, for shifting a 

 weight of 16 grammes from the scale pan to the cylinder produced no appreciable 

 difference in the rest-point of the balance, when the sensitiveness was such that one- 

 tenth of a milligramme could be detected. 



As will be seen from figs. 2, 6, and 7, the scale pans are of unusual shape. Rods 



