Chap. IV.

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tanari, and several others, which are all printed, and form almost an encyclopaedia onthe subject.

Galileo Galilei was a native of Pisa , and born in the year 1564; on one of his visits tothe beautiful Duomo, at an early age, he observed the swinging of the large chandelier,and from thence he set about constructing the first pendulum. Mathematics at this timewas at a very low condition, not only in Italy, but throughout Europe; but Euclid andArchimedes were now generally revived and studied by all who had any pretensions toscience; after Galileo had studied the writings of the latter, he published his first work,which was an essay on the hydrostatic balance, in which he proves himself thoroughlyacquainted with the principles of specific gravity. His learning then became generallyknown, and in the year 1589 he was named professor of mathematics at Pisa , where hereceived a salary of sixty crowns per annum. The phenomena of nature now formed hischief study, and he began to inquire into the mechanical doctrines of Aristotle , although hecan hardly be considered the first who impugned his high authority. Leonardo da Vinci had indulged in investigations which astonished his cotemporaries, and which were entirelyunknown to the philosophers of the time.

Galileo succeeded Moleti in the professor’s chair at Padua in 1588, and soon after tookup the study of the thermometer, which was to a certain extent the result of the Greekmathematician Hero’s contrivances. Galileo ’s tube was made of glass, the bulb having theair expelled by heat, and then filled with water ; after which the degrees were marked uponit, which indicated the expansion of the air when subjected to a change or increase of tem-perature.

In the year 1609, he invented the telescope, and three years afterwards published hisDiscourse on Floating Bodies after which he turned his attention to the sucking-pump,and when he found it would not act beyond a certain depth, he imagined some injury hadoccurred to it, and sent it to the maker to have it repaired. The maker assured him thatno pump would raise water beyond the depth of eighteen cubits, when Galileo observed,in his explanation upon this phenomenon, that a rod or column of water, when raised to theheight of eighteen cubits in a pump, its weight overpowers the attraction of the piston andthe cohesion of the particles of the fluid. lie died in 1642.

Evangelista Torricelli, born in 1608, was the pupil of Galileo at the same time with Cas-telli: he became highly interested on the subject of the pump, and made it his particularstudy ; the year after Galileo ’s death, he made an experiment upon the vacuum leftbetween the piston of a pump and the water which it raised ; after which lie filled a glasstube with mercury, hermetically sealed at one end, and closed at the other with his finger,and then inverted it into a basin of mercury, when he was surprised to find, upon the with-drawal of his finger, that the mercury stood twenty-nine inches in the tube. This indicatedat once that the column of mercury was maintained by the weight of the column of atmo-sphere, and that the thirty-three feet of water in the pipe of the sucking-pump was sup-ported in the same manner as the twenty-nine inches of mercury.

Considerable advance was made in the science of hydrodynamics by this able disciple ofGalileo . He was one of the first who showed that when water is let out at the side orbottom of a vessel, it issues with the same velocity as that which a body would acquire byfalling from the surface of the fluid to the orifice.

Torricelli gave us the means whereby we might measure the density of the atmosphere,and constructed the first barometer. But it is from his treatise, “ De Motu Graviumnaturaliter accelerato,” that we learn, for the first time, something of the complex theorywhich regulates the motion of fluids, when the orifice has a magnitude which is considerablecompared with the section of the vessel taken horizontally.

Benedetto CasteUi was born at Breccia in the year 1577 ; he was among the distinguisheddisciples of Galileo , and may be considered as the originator of a new theory of hydraulics,relative to running waters, on which subject he compiled a treatise, “ Della Mesura dell’Acque correnti,” published in 1638.

Urban VIII . having requested him to report upon the means of completing the severalworks which were then in progress to drain various parts of the papal dominions, wasthe cause of his writing the above treatise. It contains several explanations of variousphenomena relative to rivers; and he states, what some have hesitated to admit, that theabsolute velocity is proportional to the declivity of the bed, or to the height of the water.He died at Rome , 1644.

Dominico Guglielmini was born at Bologna , 1655: at the age of thirty he had so dis-tinguished himself in the various branches of science then studied, that he was appointedchief engineer of the territory belonging to the Bolognese,— a very important office, as ithad under its superintendence the confining of the numerous rivers which intersect thatcountry in all directions, and which frequently, if neglected, subjected it to inundations. Inthe year 1690, he was made professor of mathematics, and four years afterwards a newchair was created for him, under the title of that of Hydromctry, which, from that period,