consequence. The variation of force of the remontoire spring from temperature, as it only affects the pendulum through the medium of the dead escapement, is far too small to produce any appreciable effect; and it is found that clocks of this kind, with a compensated pendulum 8 feet long, and of about 2 cwt., will not vary above a second a month, if the pallets are kept clean and well oiled. No turret clock without either a train remontoire or a gravity escapement will approach that decree of accuracy. The King's Cross clock, which was the first of this kind, went with a variation of about a second in three weeks in the 1851 Exhibition, and has sometimes gone for two months without any discoverable error, though it wants the jewelled pallets which the Exchange clock has. But these clocks require more care than gravity escapement ones, and are certain to be spoilt as soon as they get into ignorant or careless hands; and consequently the gravity ones have superseded them.
The introduction of this remontoire led to another very important alteration in the construction of large clocks. Hitherto it had always been considered necessary, with a view to diminish the friction as far as possible, to make the wheels of brass or gun-metal, with the teeth cut in an engine. The French clockmakers had begun to use cast-iron striking parts, and cast-iron wheels had been occasionally used in the going part of inferior clocks for the sake of cheapness; but they had never been used in any clock making pretensions to accuracy before the one just mentioned. In consequence of the success of that, it was determined by the astronomer royal and Mr Denison, who were jointly consulted by the Board of Works about the great Westminster clock in 1852, to alter the original requisition for gun-metal wheels there to cast-iron. Some persons expressed their apprehension of iron wheels rusting; but nothing can be more unfounded, for the non-acting surfaces are always painted, and the acting surfaces oiled. A remarkable proof of the folly of the clockmakers denunciations of the cast-iron wheels was afforded at the Royal Exchange the next year. In consequence of the bad ventilation of the clock-room, together with the effects of the London atmosphere, some thin parts of the brass work had become so much corroded that they had to be renewed, and some of it was replaced with iron; for all the polished iron and brass work had become as rough as if it had never been polished at all; the only parts of the clock which had not suffered from the damp and the bad air were the painted iron work. The room was also ventilated, with a draught through it, and all the iron work, except acting surfaces, painted. Even in the most favourable positions brass or gun-metal loses its surface long before cast-iron wants repainting.
There is, however, a curious point to be attended to in using cast-iron wheels. They must drive cast-iron pinions, for they will wear out steel. The smaller wheels of the going part may be of brass driving steel pinions; but the whole of the striking wheels and pinions may be of iron. A great deal of nonsense is talked about gun-metal, as if it was necessarily superior to brass. The best gun-metal may be, and is, for wheels which are too thick to hammer; but there is great variety in the quality of gun-metal; it is often unsound, and has hard and soft places; and on the whole, it has no advantage over good brass, when not too thick to be hammered. In clocks made under the pressure of competing tenders, if the brass is likely not to be hammered, the gun metal is quite as likely to be the cheapest and the worst possible, like everything else which is always specified to be "best," as the clockmakers know very well that it is a hundred to one if anybody sees their work that can tell the difference between the best and the worst.
Turret Clocks with Gravity Escapement.
Fig. 22 is a front view of a large quarter clock of Sir E. Beckett's design, with all the wheels on the great horizontal bed, a gravity escapement, and a compensated pendulum. They are made in two sizes, one with the great striking wheels 18 inches wide, and the other 14. The striking is done by cams cast on the great wheels, about 1⅛ inch broad in the large-sized clocks, which are strong enough for an hour bell of thirty cwt., and corresponding quarters. Wire ropes are used, not only because they last longer, if kept greased, but because a sufficient number of coils will go on a barrel of less than half the length that would be required for hemp ropes of the same strength, without overlapping, which it is as well to avoid, if possible, though it is not so injurious to wire ropes as it is to hemp ones. By this means also the striking cams can be put on the great wheel, instead of the second wheel, which saves more in friction than could be imagined by any one who had not tried both. In clocks of the common construction two-thirds of the power is often wasted in friction and in the bad arrangement of the hammer work, and the clock is wearing itself out in doing nothing.
Fig. 22.—Front view of Turret Quarter Clock.
The same number of cams are given here to the quarter as to hour-striking wheel, rather for the purpose of suggesting the expediency of omitting the 4th quarter, as has been done in many clocks made from this design. It is of no use to strike ding-dong quarters at the hour, and it nearly doubles the work to be done; and if it is omitted it allows the bells to be larger, and therefore louder, because the 1st quarter bell ought to be an octave above the hour bell, if they are struck at the hour; whereas, if they are not heard together the quarters may be on the 4th and 7th of a peal of eight bells. Moreover, the repetition of the four ding-dongs can give no musical pleasure to any one.
The case is different with the Cambridge and Westminster quarter chimes on 4 bells, and the chime at the hour is the most complete and pleasing of all. It is singular that those beautiful chimes (which are partly attributed to Handel) had been heard by thousands of men scattered all over England for 70 years before any one thought of copying them, but since they were introduced by Sir E. Beckett in the great Westminster clock, on a much larger scale and with a slight difference in the intervals, they have been copied very extensively and are already almost as numerous in new clocks as the old-fashioned ding-dong quarters. Properly, as at Cambridge and Westminster, the hour bell should be an octave below the third (or largest but one) quarter bell; but as the interval between the quarters and hour is always considerable, it is practically found that the ear is not offended by a less interval. At Worcester cathedral the great 4½ ton hour bell is only 1½ notes below the 50 cwt. tenor bell of the peal, which is made the fourth quarter bell; and at some other places the quarters are the 2d, 3d, 4th, and 7th of a peal of 8, and the hour bell the 8th. Thereby you get more powerful and
