pensating 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 balancespring 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 Frod sham 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 instrumeut 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 '."

1 "I know not what such an undertaking would be even to the devil himself, but to man it would, undoubtedly, be the height of foliy."

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. 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 platinumball 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

1 The details of this dispute may be found in the "Applications of the Electric Fluid to the Useful Arts," by Mr. Alexander Bain. Lond. 1843. Professor Wheatstone's clock, &c. is described in the Phil. Trans. for 1841.

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.]

QUARANTINE.

Of all the means by which in modern times the infection of that dangerous malady, the plague, has been so much guarded against, that according to general opinion, unless the Deity render all precaution useless, it can never again become common in Europe, the most excellent and the most effectual is, without doubt, the establishment of quarantine 1. Had not history been more employed in transmitting to posterity the

1 [This opinion is not generally admitted by the most experienced medical men in this country. It is a disputed point whether the plague is even contagious; and the mass of evidence is in favour of its being so occasionally, but that the plague is usually not propagated in this manner. The disappearance of this pest from our own and most other countries of Europe is undoubtedly owing to the much greater attention paid to drainage, ventilation, and the prevention of the accumulation of filth in the streets, &c. When the peculiar atmospheric conditions upon which its diffusion depends are present, quarantine has proved insufficient to prevent its propagation.]

crimes of princes, and particularly the greatest of them, destructive wars, than in recording the introduction of such institutions as contribute to the convenience, peace, health and happiness of mankind, the origin of this beneficial regulation would be less obscure than it is at present. At any rate, I have never yet been so fortunate as to obtain a satisfactory account of it; but though I am well-aware that I am neither acquainted with all the sources from which it is to be drawn, nor have examined all those which are known to me, I will venture to lay before my readers what information I have been able to collect on the subject, assuring them at the same time, that it will afford me great pleasure if my attempt should induce others fond of historical research to enlarge it.

The opinion that the plague was brought to Europe from the East, is, as far as I am able to judge, so fully confirmed, that it cannot be any longer doubted; though it is certainly true, that every nation endeavours to trace the origin of infectious disorders to other people. The Turks think that the plague came to them from Egypt; the inhabitants of that country imagine that they received it from Ethiopia; and perhaps the Ethiopians do not believe that this dreadful scourge originated among them 1. As the plague however has always been conveyed to us from the East, and has first, and most frequently, broken out in those parts of Europe which approach nearest to the Levant, both in their physical and political situation, those I mean which border on Turkey, and carry on with it the most extensive trade, we may with the more probability conjecture that these countries first established quarantine, the most powerful means of preventing that evil. If further search be made in regard to this idea, we shall be inclined to ascribe that service to the Venetians, a people who, when the plague began to be less common, not only carried on the

1 The oldest plague of which we find any account in history, that so fully described by Thucydides, book ii., was expressly said to have come from Egypt. Evagrius in his Histor. Ecclesiast. iv. 29, and Procopius De Bello Persico, ii. 22, affirm also that the dreadful plague in the time of the emperor Justinian was likewise brought from Egypt. It is worthy of remark, that on both these occasions, the plague was traced even still further than Egypt; for Thucydides and the writers above-quoted say that the infection first broke out in Ethiopia, and spread thence into Egypt and other countries.

:

greatest trade in the Levant, but had the misfortune to become always nearer neighbours to the victorious Turks. It is also probable that the Hungarians and Transylvanians soon followed their example in this approved precaution, as the Turks continued to approach them; and this agrees perfectly with everything I have read in history.

In the first centuries of the Christian æra, it does not seem to have been known that infection could be communicated by clothing and other things used by infected persons. The Christians all considered the plague as a divine punishment, or predestinated event, which it was as impossible to avoid as an earthquake; and the physicians ascribed the spreading of it to corrupted air, which could not be purified by human art. Christians therefore gave themselves up, like the Turks at present, to an inactive and obstinate resignation in the will of God, and hoped by fasting and prayer to hasten the end of their misfortune.

The

But after the plague in the fourteenth century, which continued longer than any other, and extended over the greater part of Europe, the survivors found that it was possible to guard against or prevent infection; and governments then began to order establishments of all kinds to be formed against it. The oldest of which mention has yet been found in history, are those in Lombardy and Milan of the years 1374, 1383 and 13991.

In the first-mentioned year the Visconte Bernabo made regulations, the object of which was to guard against the spreading of the plague by intercourse and mixing with those who were infected; and with that view it was ordered, that those afflicted with this disease should be removed from the city, and allowed either to die or to recover in the open air, Those who acted otherwise were to suffer capital punishment, and their property was to be confiscated. But twenty-five years after it was strictly commanded that the clothes and things used by those who had the plague should be purified with great care and in 1383 it was forbidden under severe punishment to suffer any infected person to enter the country. These means, however imperfect, must have been attended

1 They may be found in Muratori Scriptores Rerum Italic. tom. xvi. p. 560, and xviii. p. 82, thence copied into Chenot, p. 147. See also Bocca... cio, Decamer. Amst. 1679, p. 2.