350 GEEEK SCIENCE. 



theoretical hydraulic principle : we may, therefore, easily conceive, 

 that their first attempts were very rude and imperfect, and the defects 

 of one machine were their only lessons for the construction of others 

 less imperfect ; and it was thus, by successive attempts, and reiterated 

 experiments and failures, that they were led by degrees to that state 

 of perfection to which they ultimately attained. 



Frontinus, To Sextus Julius Frontinus is commonly attributed the first theo- 



first theorist. r ^ c notions of the motion of fluids. This author was inspector of 



the public fountains of Rome, under the emperors Nerva and Trajan, 



and he left, on this subject, a work entitled ' De aquaeductibus urbis 



Romse commentarms.' 



In this treatise, the author first describes the aqueducts of Rome, 

 cites the names of those which the Romans had constructed, and the 

 dates of their constructions ; he then fixes and compares with each 

 other the measures of capacity which he employed at Rome for measur- 

 ing the products of the adjutages. Thence he passes to a description 

 of the means of distributing the waters of an aqueduct, or of a fountain. 

 On these subjects he made several correct observations ; for example, 

 he showed that the quantity of water issuing from an adjutage, did 

 not wholly depend upon its magnitude or superficies, but that the height 

 of the reservoir above it must also be considered ; a very obvious fact, 

 but yet such an one as some more recent constructors have neglected 

 to introduce into their investigations. He knew, also, that the tube 

 designed to carry off part of the water of an aqueduct, ought to have, 

 according to circumstances, a position more or less oblique with respect 

 to the course of the fluid, &c. Notwithstanding all this, however, he 

 did not exhibit a mathematical precision on this subject ; for he did 

 not know the correct law which obtained between the velocity of the 

 adjutage, and the height of the reservoir. 



No other ancient author approximated in any mariner towards a 

 theoretical view of the principle of hydraulics ; we are, therefore, com- 

 pletely justified in claiming the honour of the discovery of this science 

 as wholly due to the moderns. 



IV. PNEUMATICS. 



As the science of Pneumatics is in a great measure involved in the 

 general doctrine of the theory of fluids, many branches of its history 

 are so connected with that of hydrodynamics, that it is difficult to 

 separate them from it ; and, accordingly, many of the circumstances 

 given under the latter head will equally apply to the former. There 

 is, however, one important distinction : most of the properties of water 

 are striking and obvious, while those of air are hidden and obscure ; 

 that water is a heavy body is a fact which must have been known 

 from the earliest observations, but the gravitating properties of atmo- 

 spheric air were by no means so evident, and therefore long remained 

 a matter of doubt, even after the idea of its ponderability had been 



