308 Analyses of Books. [April 



1747 Dr. Watson found discharges through a circuit of four miles in 

 extent, two miles through wire and two through the ground, to be 

 apparently simultaneous. Mr. Wheatstone repeated a similar expe- 

 riment, substituting for the imperfect judgment of the eye, a revolv- 

 ing mirror. This instrument revolved 800 times in a second, and 

 during this time the image of a stationary point would describe 1600 

 circles ; the elongation of a spark through half a degree, a quantity 

 obviously visible, and equal to one inch seen at the distance of 10 

 feet, would therefore indicate that it exists the 1,152,000th part of 

 a second. The deviation of half a degree between the two extreme 

 sparks, the wire being half a mile in length, would indicate a velocity 

 of 576,000 miles in a second. This estimation is on the supposition 

 that the electricity passes from one end of the wire to the other : if, 

 however, the two fluids in one theory, or the disturbances of equili- 

 brium in the other, travel simultaneously from the two ends of the 

 wire, the velocity measured will be half that in the former case, or 

 288,000 miles in a second. The greatest elongation of the sparks 

 was 24", indicating a duration of about the 24,000th part of a second. 

 The general conclusions which the author draws from his experi- 

 ments are, 1st, The velocity of electricity through a copper wire 

 exceeds that of light through the planetary space. 2d, The distur- 

 bance of electric equilibrium, in a wire communicating at its extre- 

 mities with two coatings of a charged jar, travels with equal velocity 

 from the two ends of the wire, and occurs latest in the middle of the 

 circuit. 3d, The light of electricity in a state of high tension, has 

 a less duration than the millionth part of a second. 4th, The eye is 

 capable of perceiving objects distinctly which are presented to it 

 during the same small interval of time. 



Physics, 8fc. 



Mr. Hamilton's paper on a general method in Dynamics is a most 

 elaborate one. He shews that in the method formerly employed to 

 develope the laws of motion, the determination of the motion of a 

 free point in space, depends on the integration of three equations, in 

 the ordinary differentials of the second order, and the determination 

 of the motions of a system of free points attracting or repelling one 

 another, depends on the integration of a system of such equations, in 

 number threefold the number of the attracting or repelling points, 

 unless we previously diminish by unity this latter number, by consi- 

 dering only relative motions. Mr. Hamilton's method is to reduce 

 the problem to the search and differentiation of a single function, 

 which satisfies two partial differential equations of the first order, 

 and of the second degree, and every other dynamical problem respect- 

 ing the motions of any system, however numerous, is reduced, in like 

 manner, to the study of one central function. 



Mr. Ivory demonstrates that the beautiful theory of Clairaut, 

 which assumes for the foundation of its superstructure, a mass of 

 fluid in equilibrium, and that the pressure of every new stratum 

 upon the surface of which it is laid, is caused solely by the forces 



