Before concluding the enumeration of various sorts of pendulums, one suggest ed by Mr. Trougliton should be noticed, which seems worthy of trial. He pro poses that its rod should be made of bak ed potter's earth, of the same composi. tion of Wedgewood's thermometer, and furnished with a metallic cap, by which it should be sustained by the knife-edge suspension, which the celebrated Ber thoud affirms has less friction than the spring suspension The chief advantages which tubular pendulums have over those of the grid iron construction are, that they admit of being much lighter above the bob, with equal strength; that they experience less resistance from the air in their vibrations; and that they are less liable to those shakes and irregular motions in their expansions whicn the others experience: on the other nand, as the outside tube atone in them comes in contact with the air through which it passes in its vibra tions, the inner tubes can receive much less of its influence as to temperature which arises from this motion, and which Cummings has shewn to be of considera ble consequence. In Troughton's pen dulum the great difference of the masses of matter between the ascending and de scending parts must be another source of error, as the small wires of which the. latter consist will indubitably much soon er experience the influence of a change of temperature in the air, than the more bulky substance of the tubes. In this lat ter respect Chandler's tubular pendulum seems superior to Troughton's,allits parts being much more nearly of the same mag nitude.
More accurate comparative trials be tween these gridiron and tubular coma pensating pendulums, than any which have yet been made, seem, however, ne cessary to determine the superiority of either ; and the preference which many are now inclined to give the tubular con struction seems more to arise from the greater neatness of its appearance, than from any sufficientexperience of its high er merit.
That it may be superior is very possi' ble: we only aver that this has not been yet proved. But if equal apertures were made at both sides of tubular pendu lums, through all the tubes, it would ob viate the chief objection to them, by ad mitting the air to act on all their parts at once.
in the year 1803, the Society fur the Encouragement of Arts gave a premium of 20 guineas to Mr. Massey, of Hornley, in Staffordshire, for a new striking part of a clock ; the principal difference in which from the common movement was, that a pendulum about nine inches long, and which therefore vibrated pretty near ly half seconds, was used to regulate the interval of time between the strokes, in stead of the common fly wheel. The other parts of the mechanism were also of a simpler construction than those of the striking parts of the clocks in com mon use.
Mr. Prior, of Nessfield, in Yorkshire, also obtained a premium from the above mentioned society, in the same year, of 30 guineas, fur another contrivance for the striking part of a clock ; of which the advantage consisted in the simplicity of Sts structure, and the precision of its per formance, and which therefore possessed considerable merit as a piece of mechan ism ; but neither of those inventions be ing of any service to the great object of horological machinery, namely, the pre cise and accurate measurement of time, we have thought a farther description of them needless here.
Clocks being considered in this point of view, as they doubtlessly should be, no great estimation can be attached at present to those clocks on the continent, which were formerly so famous, whose chief object seems to have been to set a number of puppets in motion at stated times. Of these the clocks of Stras burgh and of Lyons were the most noted. In the former a cock claps his wings, and proclaims the hour ; and puppets, intended to represent an angel, the Vir gin, and the Holy Spirit, appear : the an gel opens a door, and salutes the virgin, and the Holy Spirit descends on her. In the clock of Lyons two horsemen en counter, and beat the hour on each other ; a door opens, and there appears on the theatre the image of the Virgin, with that of Jesus Christ in her arms ; the Magi, with their retinue, marching in order, and presenting their gifts ; two trumpeters sounding all the while, to proclaim the procession. Clocks with chimes are of the same nature with those described.
In nearly the same rank with the fore. going must be classed the• clocks made to register the motions of the heavenly bodies' they can be only considered as objects of curiosity, since in point of utility, in noting the position of the heavenly bodies, the common nautical al manacs are so superior, as to render it in some degree ridiculou's to compare them together. The clock of the royal palace at Hampton Court is one of the most noted of those which have move ments of this nature ; but other consider ations render this clock an object of great interest. According to Dr. Derham, it is the oldest English clock extant, hav ing been constructed in the year 1540, in the reign of Henry Viii. It shews the time of the day, and the motion of the sun and of the moon through all the de grees of the zodiac, together, with the day of the month, the moon's southing, and other matters. These motions are the more deserving attention, as, at the time the clock was made, Copernicus, then living, had not published his book ,• On the Revolutions of the Celestial Orbs." And besides this, the pendulum was not applied as a regulator of clocks for nearly a century afterwards.