104 



LECTURE XIII. 



racy, the most convenient test is furnished by the effects of twisting. An 

 arm or beam is suspended horizontally by a long wire, and the force 

 required to cause the beam to make one or more revolutions being ascer- 

 tained, we may divide the circle described by its extremities into as many 

 parts as we think proper, and the force required to bring the beam into 

 any position will always be proportional, without a sensible error, to the 

 magnitude of the part of the circle intercepted between the given position, 

 and that in which the arm would naturally rest. When the force is of 

 such a nature as to be capable of producing a vibration, the body on 

 which it acts being suspended by the thread of a silkworm or of a spider, 

 we may compare its magnitude with that of gravitation, by observing the 

 time required for each vibration, and determining the operation of the force 

 according to the laws of pendulums. It is in this manner that the forces 

 concerned in the effects of electricity and of magnetism have been measured 

 by Mr. Coulomb. 



LECT. XII. ADDITIONAL AUTHORITIES. 



Balances. Lahire, Hist, et Mem. de 1'Acad. ii. 9 ; ix. 42. Roberval's New 

 Balances, ibid. x. 343. Emerson's Mechanics. Troughton's Balance, Nich. 

 Jour. iii. 233. 



Steelyard. Hooke's Steelyard, Birch's History, iv. 242. Roemer's Danish 

 Steelyard. Machines Approuvees, i. 79. Pictet on Paul's Steelyard, Ph. Mag. 

 iii. 408. 



Weights and Measures. Whitehurst, An Attempt to obtain an invariable Stand- 

 ard of Length, &c. 4to, 1787. Adams (John Quincey), Report on Weights and 

 Measures. Washington, U.S. 1821. Hassler on Do. Wash. 1832. Report of the 

 Franklin Institute on Do. 1834. Pasley on the expediency of simplifying Weights, 

 &c. Lond. 1834. Clark on Weights and Measures, Westminster Review, No. 31. 

 Parliamentary Reports, fol. 1758, 1759, 1814, 1819, 1820, 1821. 



Animal Mechanics, &fc. Perrault on Animal Mechanics, Hist, et Mem. de Paris, 

 i. 181. Parent on Do. ibid, 1702, H. 95. Amontons on Moving Powers, ibid. 1703. 

 D. Bernoulli on the Muscles and Nerves, Com. Petr. i. 297. Ray, The Wisdom of 

 God manifested in the Works of Creation. Derham, Physico -Theology. 1712. 

 Paley's Natural Theology. Cuvier, Regne Animale. Bell, Animal Mechanics, Lib. 

 Useful Knowledge. Do. Bridgewater Treatise on the Hand. 



Inanimate Force. Smeaton on the Effect of Wind and Water, Ph. Tr. 1759, 

 p. 100. Reprinted, 8vo, Lond. 



LECTURE XIII. 



ON PASSIVE STRENGTH AND FRICTION. 



THE passive strength of the materials employed in the mechanical ails 

 depends on the cohesive and repulsive forces of their particles, and on the 

 rigidity of their structure. The consideration of the intimate nature c>f 

 these forces belongs to the discussion of the physical properties of matter ; 

 but the estimation of their magnitude, and of their relative value in various 

 circumstances, is of undeniable importance to practical mechanics, and 



