342 HISTORY OF MECHANICS. 



is well adapted to attract notice, since the curve described, which is 

 transient and invisible in the case of a single projectile, becomes per- 

 manent and visible when we have a continuous stream. The doctrine 

 of the motions of fluids has always been zealously cultivated by the 

 Italians. Castelli's treatise, Bella Misura delV Acque Corrente (1638), 

 is the first work on this subject, and Montucla with justice calls him 

 " the creator of a new branch of hydraulics ;" 3 although he mistakenly 

 supposed the velocity of efflux to be as the depth of the orifice from tbe 

 surface. Marsenne and Torricelli also pursued this subject, and after 

 them, many others. 



Galileo's belief in the near approximation of the curve described by 

 a cannon-ball or musket-ball to the theoretical parabola, was somewhat 

 too obsequiously adopted by succeeding practical writers on artillery. 

 They underrated, as he had done, the effect of the resistance of the air, 

 which is in fact so great as entirely to change the form and properties 

 of the curve. Notwithstanding this, the parabolic theory was employ- 

 ed, as in Anderson's Art of Gunnery (1674) ; and Blondel, in his 

 Art de jeter les Bombes (1683), not only calculated Tables on this sup- 

 position, but attempted to answer the objections which had been made 

 respecting the form of the curve described. It was not till a later 

 period (1*740), when Robins made a series of careful and sagacious 

 experiments on artillery, and when some of the most eminent mathe- 

 maticians calculated the curve, taking into account the resistance, that 

 the Theory of Projectiles could be said to be verified in fact. 



The Third Law of Motion was still in some confusion when Galileo 

 died, as we have seen. The next great step made in the school of 

 Galileo was the determination of the Laws of the motions of bodies in 

 their Direct Impact, so far as this impact affects the motion of trans- 

 lation. The difficulties of the problem of Percussion arose, in part, 

 from the heterogeneous nature of Pressure (of a body at rest), and 

 Momentum (of a body in motion) ; and, in part, from mixing together 

 the effects of percussion on the parts of a body, as, for instance, cutting, 

 bruising, and breaking, with its effect in moving the whole. 



The former difficulty had been seen with some clearness by Galileo 

 himself. In a posthumous addition to his Mechanical Bialogues, he 

 says, " There are two kinds of resistance in a movable body, one 

 internal, as when we say it is more difficult to lift a weight of a thou- 

 sand pounds than a weight of a hundred ; another respecting space, as 



3 Mont. ii. 201. 



