July 19, 1901.] 



SCIENCE. 



85 



disclosed to man objects beyond the reach 

 of his unassisted vision. The swinging of 

 a hanging lamp suggested to a thoughtful 

 man the idea of an unseen force, and Gali- 

 leo, by experiment and reasoning, discov- 

 ered the law of terrestrial gravitation and 

 first grasped the idea of force as a mechan- 

 ical agent. 



Turning his newly invented telescope 

 upon the heavens, he was the first man in 

 the world to witness the actual motion of 

 the planets and their satellites, and to prove 

 that they and the earth revolved about the 

 sun, as P37thagoras had imagined two thou- 

 sand years before, Copernicus had asserted 

 a hundred years before, and his own con- 

 temporary, Kepler, had reasoned from the 

 imperfect data then possessed, and had 

 actuall}^ formulated the laws of their mo- 

 tion in forms which the science of our day 

 confirms exactly. He was the first to con- 

 ceive the theory of transverse strains in 

 solids, but the facility of experimentation 

 with fluids diverted his attention from more 

 rigid bodies, and the fundamental principles 

 of hydraulic science were established by 

 him and by his pupils and immediate suc- 

 cessors in the fascinating studies he had 

 introduced. 



To Galileo also is due the invention of 

 the thermometer, which enabled definite 

 measurement to be made of the mysterious 

 phenomenon of heat which his great con- 

 temporary philosopher in England, Lord 

 Bacon, conceived to be '■ an expansive un- 

 dulatory motion in the particles of a body 

 whereby they tend with some rapidity 

 toward the circumference, and also a little 

 upward.' But Bacon was two hundred 

 years in advance of the physicists, and the 

 century was occupied almost exclusively in 

 the elucidation of the laws of gravitation 

 as exemplified in the action of fluids. For 

 the study of liquids and their action dem- 

 onstrated that they were, under certain 

 conditions of temperature, transformed into 



invisible and elastic substances governed in 

 general by the same laws, and also that 

 there were all around other similar sub- 

 stances which could not be condensed into 

 liquid inelastic form, but could be weighed 

 by the barometer which Torricelli invented 

 in 1643. And so the laws of gases came to 

 be investigated and formulated by Mariotte, 

 who, to aid him in his researches, invented 

 the rain gauge in 1677, and measured the 

 liquids condensed from the atmosphere. 

 The value of the data thus obtained, to the 

 hydraulic engineer, was appreciated by the 

 French engineers at once, and ever since 

 1681 records of the rainfall have been kept 

 continuously at Paris, and the practice has 

 gradually extended over the whole world. 

 But it is worthy of note that fifty years 

 after the invention of the rain gauge, 

 Belidor, in his magnificent treatise on Hy- 

 draulic Architecture (1728), the first com- 

 pendium of engineering theory and practice, 

 in treating of the sources of water supply 

 for domestic use, did not mention the rain 

 gauge or the amount of rainfall, but dwelt 

 on the divining rod as the recognized means 

 of discovering subterranean streams of 

 water. 



The close of this century saw the final 

 establishment of the law of gravitation by 

 Newton's- proof of its governing the whole 

 material universe. And any review of the 

 progress made during the century toward 

 the understanding of the laws of nature 

 would be incomplete without allusion to 

 the two great steps taken in it toward the 

 facilitating of mathematical computations, 

 the invention of logarithms at the beginning 

 of the century and of the calculus at its 

 close. 



That heat produced dynamic efiects had 

 been recognized for ages. That it destroyed 

 some solids, that it converted others into 

 different forms possessing entirely different 

 properties, that it caused the dissipation 

 and disappearance of liquids, were facts 



