Notices respecting New Books. 267 



pressure Is removed ; the theory of air-pumps, and pumps 

 for raising water; the resistance of fluids to bodies moving 

 in thorn ; and results of experiments on the resistance of 

 fluids. 



The second vohnne of this work, which, as before men- 

 tioned, is chiefly practical, commences with an introduc- 

 tion occupying about eighty pages, and comprising general 

 remarks, rules^ and directions, — on the construction and 

 simplification of machinery; on rotatory, rectilinear, and 

 reciprocating motions ; on bevel-geer, and proportioning 

 the number of teeth ; on friction and the rigidity of cords, 

 with the experiments of Vince, Coulomb, &c., and an ex- 

 ample of the power of the capstan, allowing for friction and 

 the stiffness of cords ; on water and wind as movers of ma- 

 chinery, with Smeaton's rules relative to windmills ; on the 

 strength of fired gunpowder; on steam as a mover of ma- 

 chinery, with the theorems and results of Bettancourt and 

 Dalton; and on the strength of men and horses, according 

 to the best and most accurate observations. The remaining 

 part of this volume is appropriated to the description of a 

 variety of mechanical contrivances, in number exceeding an 

 hundred and fifty. These are arranged alphabetically, that 

 they may be consulted with most convenience ; and several 

 of them relate to machines and instruments which have 

 never before been publicly described. 



In the exposition of the theory the author has generally 

 proceeded bv a series of connected propositions and depen- 

 dent corollaries ; those in each chapter having a manifest re- 

 lation to each other, and flowing naturally from the same 

 source. He has avoided the two extremes, — a fatiguing 

 prolixity of detail, which leaves nothing to be filled up by 

 the ingenuity of the pupil, and that obscurity which often 

 results from the suppression of intermediate ideas. Mr. 

 Gregory has not attempted to explain the nature of gravity, 

 impulse, or the other sources of the motion of bodies ; and 

 the reasons he assigns in his preface are perfectly satisfac- 

 tory. The general definition of the term force, in a mecha- 

 nical sense, supersedes the necessity of inquiring into the 

 essence of the various kinds of forces which may operate 



uptJU 



