. PHYSICAL AND MATHEMATICAL SCIENCES. (it

His contemporary, Hero, was distinguished for inven-tions rather curious than useful; such as ingeniouscombinations of syphons, &c. forming various kinds offountains and water-works. Such contrivances serve toillustrate the progress which was made in the knowledgeof principles, however fallacious the theories to whichthose principles were referred.

Hero adopted the idea of nature’s abhorrence of avacuum, at least as extending through any considerableor sensible portion of space. He made this limitationbecause he conceived that in the insensibly small inter-stices of bodies a vacuum does exist; and by thismeans accounted for the compressibility of matter. Hecontrived a method for the exhaustion of air out of avessel, very nearly resembling the air-pump, and prac-tically applied it to the purpose of cupping instruments.Hut to explain the effect, he deviled the long celebratedtheory of suction.

According to this theory, the upper portion of theliquid or substance is drawn up in a tube or vessel,being attracted and supported, as it were, by somemysterious and occult power, applied in the act ofsuction, whilst the next portions in succession are sus-tained by those above them; and the extent to whichsuch action can be carried is limited by the weight ofthe column suspended.

Posidonius , somewhat later than Hero, followed inthe cultivation of the same sciences. He enquired intothe nature of the tides*, and discussed the atmosphericrefraction. In this he was followed by Cleomedes , whoalso compared the magnitudes of the earth and of thesun. t

Our knowledge of all these inventions is principallyderived from the writings of Vitruvius . Some of theworks of Hero, and others of this age, appear in thevaluable collection published under the title of “ Ma-thematic! Veteres.” Others exist in manuscript in thevarious public libraries throughout Europe .

* Cic. de Nat Deor. ii.

Meteor, ci. L