Arnold’s model, though destined to perform the same office as Harrison’s, was entirely different in its construction, and was as simple as his predecessor’s was complex. By progressive stages of improvement, it was brought by the inventor himself to so high a point of perfection, that it continues to be the model followed in the construction of the best chronometers of the present day. The instruments upon which Arnold experimented are now in the possession of his successor, Mr. C. Frodsham, and show the gradual progress of advancement made in the escapement, &c., until he arrived at that beautiful, yet simple, detached escapement, which is still followed, and known under the name of the Arnold escapement. He was the first watchmaker who introduced jewelling into watches and clocks, and in 1771 he applied ruby pallets to the two clocks of the Royal Society by Graham and Smeaton, and likewise to the transit clock by Graham at the Royal Observatory, Greenwich. In 1776 Arnold achieved what was unquestionably his greatest work, viz. the invention of the cylindrical spring and compensation balance, and their application in the chronometer, which is the name that Arnold then first employed to designate his timekeepers. This ingenious and valuable discovery introduced a new æra in chronometry. Each part of the machine under the new arrangement performed unchecked the office assigned to it. The escapement was completely detached, except at the moment of discharge and giving impulse; the balance-spring, no longer interfered with in corrections for temperature (as formerly) by the compensating curb, became a free agent and the generator of motion, in which state only it is capable of being perfectly isochronized; the balance, by its expansion and contraction, varied its inertia according to the varied tension of the balance-spring by its increased or diminished elastic force in changes of temperature, while the office of the main-spring was reduced to that of a simple maintaining power. This beautiful discovery, together with the law of isochronism and other important improvements in the modification of the compensation balance, procured for him and his son John Roger Arnold the reward from government of the sum of £3000. The accuracy with which chronometers keep time is truly astonishing: in 1830 two chronometers constructed by Mr. Charles Frodsham were submitted for public trial at the Royal Observatory, Greenwich, for twelve months, and were observed daily. One of them made an extreme variation of 86-hundredth parts of a second, and the other of 57-hundredths only; but even this degree of accuracy, surprising as it is, is surpassed by the performance of his best astronomical clocks. It is therefore highly honourable to the English artists, that by their ingenuity and skill they have accomplished the great object which had occupied the attention of the learned of Europe for nearly 300 years, namely, the means of discovering the longitude at sea. It is not a little singular that Sir Isaac Newton suggested the discovery of the longitude by the aid of an accurate timekeeper.
If we go back to the period of Philip III. of Spain, we shall then see the interest and importance attached to this great discovery. As early as 1598, this monarch offered a reward of 100,000 crowns to any person who should discover the means of finding the longitude of a ship at sea; but what was the opinion then entertained of the nature of the task to be accomplished by means of the balance-watches then in use, may be gathered from an expression of Morin, who wrote about the year 1630, and who in speaking to the Cardinal Richelieu of the difficulty of constructing an instrument which should keep time to the requisite degree of accuracy for that purpose, is reported to have said, “Id verò an ipsi dæmoni nescio, homini autem suscipere scio esse stultissimum[1122].”
We have not said much on the beautiful discovery of the law of isochronism, of the balance-spring, on which the higher adjustment of clocks and watches so entirely depend, as an elaborate essay on this subject by Charles Frodsham, F.R.A.S., is in the hands of the publisher, and will shortly be circulated.
Some very ingenious contrivances have within the last few years been effected in the application of the electric fluid as a source of motive power to clocks and chronometers, and they offer peculiar advantages in the great simplicity of the apparatus in which wheels are dispensed with, hence friction is reduced to a minimum. Their invention is a subject of dispute between Professor Wheatstone and Mr. Alexander Bain[1123]. We shall briefly describe Mr. Bain’s clock. His source of electricity is obtained by fixing galvanic plates in moist earth. The clock consists of a pendulum, the bob of which vibrates between the poles of two permanent magnets, the opposite poles of which face one another. A small platinum-ball is affixed to the upper part of a small brass stem, which is free for lateral motion, being fastened below to a light spindle carried by the upper part of the pendulum-rod. A wire coated with silk is attached to the lower end of the suspension-spring of the pendulum. It is led down the back of the rod (which is composed of wood) and then coiled longitudinally in many convolutions around the edge of the bob in a groove. It is then taken up the back of the rod and terminates in the bearings of the spindle. The pendulum is suspended from a metal bracket fixed to the back of the case, and to which one of the poles of the battery is attached. Two pins are fixed horizontally, parallel with the platinum-ball, leaving space for its lateral motions, and at such a distance that the ball alternately comes into contact with each pin, when the pendulum has reached the opposite extremity of its arc. The other pole of the battery is placed in contact with the metal bracket which supports one of the pins. As long as the platinum-ball rests on the pin projecting from the pin-bracket to which the second pole of the battery is attached, a constant current of electricity is established and passes through the earth, the plates and the wires. But when the pendulum is set in motion by being drawn on one side, the point of support of the rod carrying the platinum-ball is thus moved to the same side, hence the centre of gravity of the platinum-ball being removed beyond its base, it falls upon the opposite pin. This motion of the ball lets on and cuts off the flow of electricity, at or near the ends of the vibrations of the pendulum, so that the convoluted wire of the bob is alternately attracted and repelled by the magnets at the proper points of its vibrations, and thus a continual motion is kept up. Mr. Bain has also contrived arrangements by which a great number of clocks may be worked simultaneously or in rotation; as also by which ordinary clocks may be made to keep time. The latter are effected by transmitting a current of electricity once in every four hours from a regulating clock. As the details connected with these valuable contrivances can hardly be followed without figures, we must rest satisfied with referring the reader to Mr. Bain’s work, before cited.]
FOOTNOTES
[1089] This article was written by the Hon. Daines Barrington. It is here given with the addition of Professor Beckmann’s notes, which are distinguished by the initials of his name.
[1090] Dante, Paradiso, c. x.
[1091] Selden, in his preface to Hengham.
[1092] Mic. 2 Ric. III.
[1093] We find that this clock was considered, during the reign of Henry VI., to be of such consequence, that the king gave the keeping of it, with the appurtenances, to William Warby, dean of St. Stephens, together with the pay of sixpence per diem, to be received at the Exchequer. See Stow’s London, vol. ii. p. 55. The clock at St. Mary’s, Oxford, was also furnished in 1523, out of fines imposed on the students of the university.