AND SURGICAL INSTRUMENTS.

North, Nobtii Central, and South Central Galleries.

421

Kini

gdom.j

willing; one in front of the cross gallery at the east™ °f the central aisle, and the other in front of theouth gallery of the transept. All the dials are regu-a nil by one pendulum, represented in fig. 5, and which

Fig. 5.

s suspended from a triangular frame, g, resting on a bed-We, to which it is secured by screw bolts in the ordinaryh‘ f ^ le pendulum has a mercurial compensation for^ e at and cold, and is kept in motion by electro-magnetisma method entirely different to any previously invented.„ s tead of applying directly the attractive and repulsivet o rces of electro-magnetism to the pendulum, accordingall the previous methods, the power of an electro-magnet is here employed to bend a spring to a certainin ex tent, the reaction of which gives the necessaryPulse to the pendulum, by which means the variationsUo' 1C • are contin ually taking place in the batteries haveeffect on the time measured. The arrangement of theitil’- S* v ing the impulse is represented at s (fig. 51,st 'J'uich b is the impulse spring, consisting of a shorttiol s P r * n §’ to which are attached two arms, c and d, athoi v an S^ es to each other; e is the detent, or catch, forf . wng this spring when bent by the action of the magnet;

the pendulum-rod carrying the two screws, h and i,. u ch may be called the impulse and discharging pallets.PaV * 16 P endu lum vibrates to the left, the discharging-K p 1 pressing against the perpendicular arm of thec tent > e, forces it into the position indicated by thempT- 6 ^ 1 ' nes ’ tlle impulse spring is thus liberated and im-d lately falls against the impulse pallet h. As the pen-el returns to the right, the impulse spring by itsPow C ‘ ty urges the pendulum forward with the exactli m - er required to continue its vibration. The spring issere ■ in its mutiou by a screw, o, by screwing or un-aa g Wlu g which the length of the stroke of the spring,g r Cons equently the power may be regulated to thethe ■ ^ uicety. The pendulum continuing its motion tocomes in contact with a slight spring tippedh at , Plutina, which completes the circuit of the galvanicp^ ’y through the coils of the electro-magnet, which is■v e rt„; tWmediately underneath the bed-plate in an in-Plate a P? s Wi°n, the poles of which pass through the bed-p] a • n arma ture, k, consisting of a flat bar of iron, ishori ? Imme <Iiately above the poles, being attached to a^reelv Dta ^ arm at right angles thereto, which arm movesa brain 11 an ax * s > x > properly supported at either end onarm a | ' 0° the opposite side of the axis is anotherlong er !? Projecting at right angles, but considerablyHexm n *- n t * le ^ rst- The use the second arm, in con-from au uiljustable weight, is to raise the armature

of e ] e e .P?les of the magnet. When, therefore, a currenttrhicjj nc ity is made to pass through the coils of wiretovp ard Su U oun<i the magnet, the armature is attractedtheadii ,, C P°l es , and consequently the long arm withIt i s e ystable weight is elevated in the opposite direction.ia g th < M that this arm cannot be raised without lift-urm, d, of the impulse spring, b, bending the

impulse spring, and locking the upper end of the arm, c,on the detent, ready for the next impulse.

[A point of importance in the construction of thisclock is the method of making and breaking contact forthe electric currents. When the circuit is broken, aspark is seen to pass between the points of contact. Thecontinued action of this spark causes the points betweenwhich it passes to become oxidised; and as the metallicoxides are non-conductors of electricity, it follows thatthe passage of the electricity will be thereby interferedwith and prevented.

In the first clock constructed, a piece of steel-wire wmsused as a break spring, touching against the side of thependulum-rod; but the points of contact oxidised sorapidly, that the clock would not go for more than a fewdays without stopping. The steel spring was then re-moved, and one of gold substituted, and a small plateof gold was soldered to the side of the pendulum-rod.The difficulty appeared to have been entirely overcome;but in six weeks the quantity of electricity passing wasconsiderably reduced, and at the end of two months theclock stopped.

Platina was next tried, in the same manner as the gold,in a new clock, completed in 1848, since which time thepoints of contact have never required cleaning, the cir-cuits being completed at the present time with as muchcertainty as when the clock was first put together. Thesepoints of contact present some peculiarities when exa-mined with a lens. With metals having a great affinityfor oxygen, a black spot forms immediately at the pointof contact; while in the case of platina the immediatepoint of contact remains perfectly clean, a rim of blackforming around it. This black rim has been found to bequite capable of conducting electricity. The probableconclusion, therefore, is, that the black rim is platina ina very fine state of division, and not an oxide of themetal.

The battery best adapted for these clocks is that ofMr. Smee, both on account of its simplicity and the easewith which it is recharged. The amalgamated zinc em-ployed in this battery is subject to rapid local action, bythe quantity of impurity which it contains, consistingusually of lead, iron, copper, tin, and cadmium; allthese metals having less affinity for oxygen than zinc,become negative when immersed in dilute acid, and forma voltaic circle with the surrounding particles of zinc.The use of amalgamation is to stop this action, which,when the amalgamation is fresh, it accomplishes; but ina few days the local action again commences, and in-creases until the acid is neutralised, or the whole of thezinc dissolved.

This may be obviated by standing the zinc plate loosein the jar, resting on the bottom, and pouring in an ounceor two of mercury. This plan is found to answer remark-ably well; the quicksilver soaking up the zinc plate keepsit thoroughly amalgamated. The zinc may be melted,and after being mixed with mercury, cast in moulds; thequicksilver would then form one of the impurities pre-sent : and should local action take place at any one point,the solution of the zinc would not only liberate the othermetals present, but liberate at the same time sufficientquicksilver to cover them, and stop the local action.

It is well known that the zinc of a battery is actedupon more severely at the surface of the liquid than else-where, by which the lower part is wasted. This appearsto depend on the presence of oxygen; for it does not goon, where the battery has been enclosed in a bottle to

collect the hydrogen evolved. A double advantage results

•