PHYSICAL SCIENCE. 



XVI 1 



of bigotry and envy, with which he continued to 

 be harassed throughout the rest of his life. 



By means of the inclined plane he succeeded 

 in demonstrating that the motion of a falling 

 body, is a uniformly accelerated motion. His 

 next step was to determine the path of a heavy 

 body, when obliquely projected. He showed 

 this path to be a parabola. The theory of the 

 inclined plane showed that if a circle be placed 

 vertically, the chords of the different arches ter- 

 minating in the lowest point of the circle are all 

 descended through in the same space of time. 

 But when Galileo applied this to account for 

 -.great and small vibrations of a pendulum being 

 performed in the same time, he fell into an 

 error, which was first completely rectified by 

 Huygens. 



In the list of the mechanical discoveries of 

 Galileo, may be placed the knowledge of the 

 existence of the law of continuity, and the em- 

 ployment of it as a principle in his reasonings on 

 the phenomena of motion. 



Torricelli, the pupil and friend of Galileo, 

 discovered a remarkable property of the centre 

 of gravity, and a general principle with respect 

 to the equilibrium of bodies. It is this : If 

 there be any number of heavy bodies connected 

 together, and so circumstanced, that by their 

 motion their centre of gravity can neither ascend 

 nor descend, these bodies will remain at rest 



Descartes, whose reputation was so great, and 

 his pretensions so high, likewise treated of 

 motion ; but in general his opinions were so 

 erroneous or unsound, that in the present rapid 

 sketch they are not entitled to notice. 



The laws which regulate the collision of 

 bodies remained unknown, till they were recom- 

 mended by the Royal Society to the particular 

 attention of its members. Three papers soon 

 appeared, in which these laws were correctly 

 laid down, though no one of the authors had any 

 knowledge of the 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 

 re-action, and the maxim that the same force 

 communicates to different bodies velocities 

 which are inversely as their masses, are the prin- 

 ciples on which these investigations are founded. 



Huygens was the first person who explained 

 the true relation between the length of the pen- 

 dulum, and the time of its least vibrations, and 

 who gave a rule by which the time of the recti- 

 linear descent through a line equal in length to 

 the pendulum, might from thence be deduced. 

 He next applied the pendulum to regulate the 

 motion of a clock, and gave an account of its 



construction, and the principles of it in his 

 Horologium Oscillatorium, about the year 1670, 

 though the date of the invention goes as far back 

 as 1656. Lastly, he showed how to correct the 

 imperfection of a pendulum, by making it to vi- 

 brate between cycloidal cheeks, in consequence of 

 which its vibrations, whether great or small, 

 become precisely equal. 



The appearance of the Principia of Newton, 

 in 1687, effected an almost total revolution in 

 mechanics, by giving new powers and new direc- 

 tions to its researches. The composition of 

 forces was treated independently of the compo- 

 sition of motion. From the equality of action 

 and re-action, it was inferred, that the state of the 

 centre of gravity of any system of bodies is not 

 changed by the action of these bodies on each 

 other. From this it follows, that the quantity of 

 motion existing in nature, when estimated in any 

 given direction, continues always of the same 

 amount. 



But the reduction of questions concerning 

 force and motion, to questions of pure geometry, 

 and the mensuration of mechanical action by its 

 nascent effects, constitute the great glory of the 

 Principia. A transition was there made from 

 the consideration of forces acting at stated inter- 

 vals, to that of forces acting continually ; and 

 from forces constant in quantity and direction, to 

 those that converge to a point, and vary as any 

 function of the distance from that point; the 

 proportionality of the areas described about the 

 centre of force, to the times of their description, 

 the equality of the velocities generated in de- 

 scending through the same space by whatever 

 rout ; the relations between the squares of the 

 velocities produced or extinguished ; and the 

 sum of the accelerating or retarding forces, com- 

 puted with a reference not to the times during 

 which, but to the distance over which they have 

 acted ; may be mentioned as a few of the dyna- 

 mical and mechanical discoveries contained in 

 that immortal work. 



Leibnitz, the rival and antagonist of Newton, 

 made some improvements in mechanical science, 

 which though not capable of being compared 

 with the profound discoveries of Newton, yet 

 require to be noticed. Leibnitz seems to have 

 been the first who announced in general terms 

 the principle of a stifficient reason. This prin- 

 ciple however was not new, for it had been as- 

 sumed by Archimedes, and employed by Galileo. 

 Stript of the metaphysical garb in which it was 

 placed by Leibnitz, it means nothing more, than 

 that nothing exists in any state, without a reason 

 determining it to be in that state rather than in 

 any other. This principle, though true in itself 

 in a limited sense, was rather brought into dis- 

 credit by the way in which Leibnitz employed 



