564 NEGEETTI AND ZAMBEA, HOLBOEN YIADUCT, E.G., 



" By a beautiful application of this principle, Wheatstone contrived an apparatus by 

 which he demonstrated that the light of the electric discharge does not last the one-millionth 

 part of a second of time. His plan was to view the image of a spark reflected from a plane 

 mirror, which, by means of a train of wheels, was kept in rapid rotation on a horizontal axis. 

 The number of revolutions performed by the mirror was ascertained to be 800 in a second, 

 during which time the image of a stationary point would describe 1,600 circles, because 

 from the laws of reflection the image of an object in a revolving mirror has twice the 

 angular velocity of the latter, and the elongation of the spark through half a degree would 

 indicate that it existSy-g-J^oth part of a second. A jar was discharged through a copper 

 wire half a mile in length, interrupted both in the middle and also at its two extremities, 

 so as to give three distinct sparks. The deviation of half a degree between the two extreme 

 sparks would indicate a velocity of 567,000 miles in a second. This estimated velocity is 

 on the supposition that the electricity passes from one end of the wire to the other ; it 

 however, according to the two fluid theory, the two electricities travel simultaneously from 

 the two ends of the wire, the two external sparks will keep their relative positions, the 

 middle one alone being deflected, and the velocity measured will be only one-half that in 

 the former case, viz., 288,000 miles in a second." 



There are, however, great discrepancies in the different measurements which have 

 been recorded of the velocity of Electricity, thus : 



Walker (America) with Telegraph Iron wire makes it 18,780 miles per second. 

 O'Mitchell (America) 28,524 



Fizeau and Gonnelle, Copper Wire, make it . . 112,680 



Iron Wire . . 62,600 



* Astronomers of Greenwich and Brussels, Copper, ) 700 



London and Brussels telegraph, make it . ) ' 



Astronomers of Greenwich and Edinburgh, Copper, I ,- /.^ 

 London and Edinburgh telegraph, make it . j * , 



NOAD. 



LAWS OF FALLING BODIES. 



"The Velocity which is communicated to a "body falling freely by Gravity. Bodies 

 falling freely near the earth's surface, have communicated to them equal additions of velocity 

 in equal times ; and since, by the first law of motion, none of these increments of the velocity 

 are lost, but all accumulated in the falling body, it follows that its actual amount at any 

 time must be proportioned to the time during which the body has fallen. If, for instance, 

 a body has fallen through ten seconds, since in each second the attraction of the earth wil 1 

 have communicated to it the same addition of velocity, and since all these additions of 

 velocity will be retained in it, its actual velocity must be ten times that which it would 

 have had after falling one second. 



" The velocity which gravity thus communicates to a falling body in each second of time 

 near the earth's surface is 32 feet, so that after falling five seconds, its velocity will be five 

 times this amount ; after ten seconds ten times this amount ; and so on. The velocity is so 

 great, that it would never have been possible to ascertain its amount by direct observations 

 on the fall of heavy bodies. 



" Could we, however, by any contrivance, neutralise the gravitating tendency of a body 

 to any known amount ; reduce it, for instance, to one-half or one-tenth or one-hundredth of 

 what it was ; since we should diminish the velocity communicated to it in each second pre- 

 cisely to the same amount, we might thus render its motions so slow that they might be 

 observed and measured, we might thus find the amount of the additional velocity actually 

 communicated to it in each second, and this multiplied by the known number of times by 

 which we had previously diminished the force of its gravity, would give us the velocity which 

 that fall would communicate in each second when undiminislied . This is the object of 

 Attmood's Machine:' (Page 492) Illustrations of Mechanics, Moseley. 



* Athenceum, January 14th, 1854, 



