Thus, the obligation which the theory of has to this philosopher, consists in his having pointed out the nature of centrifugal force, and ascribed that force to the true cause, the inertia of body, or its tendency to uniform and rectilineal motion.
The laws which actually regulate the collision of bodies remained unknown till some years later, when they were recommended by the Royal Society of London to the parti cular attention of its members. Three papers/soon appeared, in which these laws were all correctly laid down, though no one of the authors had any knowledge of conclusions obtained by the other two. The first of these was read to the Society, in November 1668, by Dr Wallis of Oxford ; the next by Sir Christopher Wren in the month following, and the third by Huygens in January 1669. The equality of action• and reaction, and the maxim, dna the same force communicates to different bodies velocities which are inversely as their masses, are the principles on which these investigations are founded.
The ingenious and profound.mathematician last mentioned is also the first who explain. ed the true relation between the length of a pendulum, and the time of its least vibrations, and gave a rule by which the time of the rectilineal descent, through a line equal in length to the pendulum, might from thence be deduced. He next applied the pendulum to re gulate the motion of a clock, and gave an account of his construction, and the principles of it, in his Horolosium Oscillatorium, about the year 1670, though the date of the inven tion goes as far back as 1656.' Lastly, He taught how to correct the imperfection of a pendulum, by making it vibrate between cycloidal cheeks, in consequence of which its vi brations, whether great or small, became, not approximately, but precisely of equal du 'ration.
Robert Hooke, a very celebrated English mechanician, laid claim to the same applica tion of the pendulum to the clock, and the same use of the cycloidal cheeks. There is, however, no dispute as to the priority of Huygens' claim, the invention of Hooke being as late as 1670. Of the cycloidal cheeks, he is not likely to have been even the second in ventor. Experiment could hardly lead any one to this discovery, and he was not sufficient ly skilled in the mathematics to have found it out by mere reasoning. The fact is, that though very original and inventive, Hooke was jealous and illiberal in the extreme ; he ap propriated to himself the inventions of all the world, and accused all the world of appro priating his.
IC has already been observed, that Galileo conceived the application of the pendulum to the clock earlier, by several years, than either of the periods just referred to. The inven tion did great honour to him and to his two rivals ; but that which argues the most pro found thinker, and the most skilful mathematician of the three, is the discovery of the re lation between the length of the pendulum and the time of its vibration, and this discovery belongs exclusively to Huygens. The method which he followed in his investigation, availing himself of the properties of the cycloid, though it be circuitous, is ingenious, and • highly instructive.
An invention, in which Hooke has certainly the priority to any one, is the application of a spiral spring to regulate the balance of a watch. It is well known of what practical utility this invention has been found, and how much it has contributed to the solution of the problem of finding the longitude at sea, to which not only he, but Galileo and Huygens, appear all to have had an eye.
In what respects the theory of motion, Huygens has still another strong claim to our notice. This arises from his solution of the problem of finding the centre of oscillation of a compound pendulum, or the length of the simple pendulum vibrating in the same time with it. Without the solution of this problem, the conclusions respecting the pendulum were inapplicable to the construction of clocks, in which the pendulums used are of neces ay compound. The problem was by no means easy, and Huygens was obliged to intro duce a principle which had not before been recognised, that if the compound pendulum, after descending to its lowest point, was to be separated into distinct and uncon nected with one another, and each left at liberty to continue its own vibration, the com mon centre of gravity of all those detached weights would ascend to the same height to which it would have ascended had they continued to constitute one body. The above principle led him to the true solution, and his investigation, though less satisfactory than those which have been since given, does great credit to his ingenuity. This was the most difficult mechanical inquiry which preceded the invention of the differential or fluxionary calculus.