1 62 HISTORY OF SCIENCE. 



University of Pisa, and as yet ignorant of mathematics, medicine being 

 his intended profession, he happened one day to observe in the cathe- 

 dral of that city the swinging of a chandelier hanging from the lofty 

 roof. The young philosopher was struck by a circumstance which 

 would have seemed to an ordinary observer a thing of no importance 

 whatever. That was, that the oscillation of the chandelier, whether 

 of smaller or greater extent, appeared to occupy equal intervals of 

 time. He put this observation to the test while still in the church by 

 counting the beats of his pulse occurring during the swing of the chan- 

 delier. After having verified the correctness of his observation by 

 repeated experiments, it occurred to him that oscillation of a pen- 

 dulous body might conversely be advantageously employed by physi- 

 cians for comparing the rates of the pulses of theii*patients. He con- 

 structed the first pendulum when he put this idea into practice. His 

 arrangement was simply a weight fastened to a string, which was held 

 in the hand at such a point that the vibrations of the weight coincided 

 with the beats of the patient's pulse, when the length of the string was 

 ascertained by a graduated rule. The scale and cord were afterwards 

 connected in one instrume.nt, which under various forms soon came 

 into common use among physicians,, and was called a pulsilogia. 

 Though Galileo thus proposed the pendulum as a time-measurer, and 

 afterwards employed it for astronomical purposes, its application to 

 clocks was not due to him. The merit of having first made this ap- 

 plication has been claimed for several persons, but is generally attri- 

 buted to Huyghens. Galileo, however, suggested in 1637 a plan by 

 which the pendulum might be made to count its own vibrations. The 

 suggestion was that a projection from the moving part of the pen- 

 dulum should in passing touch the tooth of a very light wheel, so as 

 at each vibration to move the wheel onwards by a known fraction of 

 its circumference. 



Galileo was the first to investigate the laws of motion and force by 

 a union of experimental researches with mathematical reasoning. He 

 proved experimentally that a body falling from rest passes through 

 spaces which are proportional to the square of the time of descent. 

 Here is the account of his experiments : " In a plank of wood about 

 twelve yards long, half a yard broad one way and three-quarters the 

 other, we made upon the narrow side or edge a groove of little more 

 than an inch wide : we cut it very straight, and to make it very smooth 

 we glued upon it a piece of vellum, polished and smoothed as exactly 

 as possible ; and in that we let fall a very hard, round, and smooth 

 brass ball, raising one of the ends of the plank a yard or two at plea- 

 sure above the horizontal plane. We observed, in the manner that I 

 shall tell you presently, the time which it spent in running down, and 

 repeated the same observation again and again to assure ourselves of 

 the time, in which we never found any difference ; no, not so much 

 as the tenth part of one beat of the pulse. Having made and settled 



