20. It has been doubted whether this demonstration of Stevinus be satisfactory, and also whether the theorem had not been proved in a different manner by an earlier writer. The claims of Stevinus, however, have very recently been maintained by an author of high reputation.[1371] The Statics of this ingenious mathematician contain several novel and curious theorems on the properties of other mechanical powers besides the inclined plane. But Montucla has attributed to him what I cannot find in his works. “In resolving these questions (concerning the ratios of weights on the oblique pulley), and several others, he frequently makes use of the famous principle which is the basis of the Nouvelle Mécanique of M. Varignon. He forms a triangle, of which the three sides are parallel to the three directions, namely, of the weight and the two powers which support it; and he shows that these three lines express this weight and these powers respectively.”[1372] Playfair, copying Montucla, I presume, without looking at Stevinus, has repeated this statement, and it will be found in other modern histories of physical science. This theorem, however, of Varignon, commonly called the triangle of forces, will not, unless I am greatly mistaken, be discovered in Stevinus. Had it been known to him, we may presume that he would have employed it, as is done in modern works on mechanics, for demonstrating the law of equilibrium on the inclined plane, instead of his catenarian hypothesis, which is at least not so elegant or capable of so simple a proof. It is true that in treating of the oblique pulley, he resolves the force into two, one parallel, the other perpendicular to the weight; and thus displays his acquaintance with the composition of forces. But whether he had a clear perception of all the dynamical laws, involved in the demonstration of Varignon’s theorem, may possibly be doubtful; at least, we do not find that he has employed it.
[1371] Playfair’s Dissertation. Whewell’s Hist. of Inductive Sciences, ii. 11, 14. Compare Drinkwater’s Life of Galileo, p. 83. The reasoning which Mr. W. suggests for Stevinus, whether it had occurred to him or not, may be very just, but borders, perhaps, rather too much on the metaphysics of science.
[1372] Montucla, ii. 180.
Hydrostatics. 21. The first discovery made in hydrostatics since the time of Archimedes is due to Stevinus. He found that the vertical pressure of fluids on a horizontal surface is as the product of the base of the vessel by its height, and showed the law of pressure even on the sides.[1373]
[1373] Montucla, ii. 180.
Gilbert on the Magnet. 22. The year 1600 was the first in which England produced a remarkable work in physical science; but this was one sufficient to raise a lasting reputation to its author. Gilbert, a physician, in his Latin treatise on the Magnet, not only collected all the knowledge which others had possessed on that subject, but became at once the father of experimental philosophy in this island, and by a singular felicity and acuteness of genius, the founder of theories which have been revived after the lapse of ages, and are almost universally received into the creel of the science. The magnetism of the earth itself, his own original hypothesis, nova illa nostra et inaudita de tellure sententia, could not, of course, be confirmed by all the experimental and analogical proof, which has rendered that doctrine accepted in recent philosophy; but it was by no means one of those vague conjectures that are sometimes unduly applauded, when they receive a confirmation by the favour of fortune. He relied on the analogy of terrestrial phenomena to those exhibited by what he calls a terrella, or artificial spherical magnet. What may be the validity of his reasonings from experiment it is for those who are conversant with the subject to determine, but it is evidently by the torch of experiment that he was guided. A letter from Edward Wright, whose authority as a mathematician is of some value, admits the terrestrial magnetism to be proved. Gilbert was also one of our earliest Copernicans, at least as to the rotation of the earth;[1374] and with his usual sagacity inferred, before the invention of the telescope; that there must be a multitude of fixed stars beyond the reach of our vision.[1375]
[1374] Mr. Whewell thinks that Gilbert was more doubtful about the annual than the diurnal motion of the earth, and informs us that in a posthumous work he seems to hesitate between Tycho and Copernicus. Hist. of Inductive Sciences, i. 389. Gilbert’s argument for the diurnal motion would extend to the annual. Non probabilis modo sed manifesta videtur terræ diurna circumvolutio, cum natura semper agit per pauciora magis quam plura, atque rationi magis consentaneum videtur unum exiguum corpus telluris diurnam volutationem efficere quam mundum totum circumferri.
[1375] l. 6. c. 3. The article on Gilbert in the Biographie Universelle is discreditable to that publication. If the author was so very ignorant as not to have known anything of Gilbert, he might at least have avoided the assumption that nothing was to be known.
Sarpi, who will not be thought an incompetent judge, names Gilbert with Vieta, as the only original writers among his contemporaries. Non ho veduto in questo secolo uomo quale abbia scritto cosa sua propria, salvo Vieta in Francia e Gilberti in Inghilterra. Lettere di Fra Paolo, p. 31.
Sect. II.—On Natural History.