95 HISTORY OF SCIENCE. 



substances known to exhibit the property of attracting light bodies when 

 rubbed. Gilbert found that most gems possessed the same property, 

 and so also did glass, sulphur, wax, resin, talc, alum, and many other 

 bodies. He also observed that the substances attracted might be 

 metals, stones, earths, liquids, etc. Though these observations did 

 not conduct Gilbert to any general law, they sufficed to give the initial 

 impulse to a new science, and we shall in the sequel see how rapidly 

 this science developed. In Gilbert's work the laws of magnetic po- 

 larity are clearly demonstrated, the various forms of magnets are dis- 

 cussed, the positions of the poles determined, the armatures of load- 

 stones (Fig. 38) considered, the grouping of iron filings about the poles 

 of magnetic bars (Figs. 39 and 40) examined, and many other ques- 

 tions treated by purely inductive and experimental methods. 



The science of Mechanics had its foundations laid in the century we 

 are now considering. The principle of the resolution and composition 

 of motions was distinctly referred to in an astronomical work by 

 JEROME FRACASTOR, who shows that bodies have a tendency to move 

 towards the centre of the earth in a straight line, and when they are 

 projected 'in any direction transverse to that straight line they still 

 have the same downward motion as if they were simply falling to the 

 earth. The conditions of equilibrium with regard to the lever were 

 known to the ancients, and this is, perhaps, the only one of the me- 

 chanical powers of which they established the theory. The inclined 

 plane seems to have presented greater difficulties, and its principle 

 was not demonstrated until the close of the sixteenth century, after 

 the problem had been unsuccessfully attempted by Cardan and by 

 Ubaldi in 1577. The latter considers chiefly the wedge; and in com- 

 paring the direction in which it tends to produce motion in the body 

 acted upon with that in which the motion actually takes place, he 

 says that there is a certain "repugnance" between these directions 

 which is greater as the angle of the wedge is more obtuse, and hence 

 he infers that the more acute is the angle of the wedge, the more 

 easily will it produce its effect. He does not, however, find the exact 

 proportion of the force. He correctly observes that the screw may 

 be considered as virtually a wedge wrapped 

 round a cylinder. The first person who really 

 solved the problem of the theory of oblique 

 forceswasSTEviNusoFBRUGEs(i55o 1633), 

 whose investigations entitle him to be con- 

 sidered the founder of the science of Statics. 

 FIG. 42. jj e correc tly deduced the ratio of the power 



to the weight on the inclined plane by a method 



of reasoning which was at once original, simple, and ingenious. Pie 

 supposed a perfectly flexible uniform chain to be placed on an inclined 

 plane as represented by the dotted line in Fig. 42. The chain is 

 supposed to form an endless band and to have absolute freedom of 



