NATURAL PHILOSOPHY. 219 



a new application of it, the most lofty in its object, and the most inge- 

 nious in its nature, it was made to render visible the rotation of the 

 globe. Perhaps it was in reflecting on this elegant experiment, that I 

 have been led to the thought of another kind of application, which, 

 without participating in its elevated character, recommends itself 

 by having in view an end of real utility, that end having reference to 

 the determination of velocities. 



The question with which we are dealing is, the application of the 

 pendulum to the measure of the real velocity of vehicles ; that is, their 

 velocity referred to the center of the earth. In everything that con- 

 cerns navigation, the importance of such an object will be immediately 

 perceived. There exist, indeed, several precise and convenient 

 means of measuring the speed of trains on railways for example, 

 where the point of support between the wheel and the rail remains 

 fixed (as distinguished from slipping), it is sufficient to be able to as- 

 sign the rapidity of the rotation of the wheels. But at sea the case is 

 very different. The sea is subject to currents, of which the extent, 

 the direction and the velocity are but imperfectly known. The sus- 

 taining point is thus itself transported in one direction or the other, 

 and the log, the only elementary and non-astronomical rucaus of 

 measuring the speed of ships, gives, under any circumstances, only 

 the difference between it and that of the surface of the water. On 

 the contrary, it is possible to ascertain the real speed of the vessel by 

 the indications cf the pendulum, of which I am about to endeavor to 

 explain the principle. 



Suppose that a pendulum beating, say half-seconds, and carrying a 

 bob weighing a few grammes (a gramme is equal to 15.44597 grains 

 Troy) be suspended in such a manner that its plane of oscillation may 

 be parallel to the axis of progression of the vehicle. If the pendulum 

 be vertical, the bob and the point of suspension being subjected to the 

 same velocity, it would remain in the same state for an indefinite period 

 making abstraction of irregularities of motion, and derangements of 

 various kinds. But if, by a small impulsion given by the hand in a 

 direction contrary to that of the motion of the vehicle, the bob be made 

 to recede a little 'from the verticle, the velocity of the point of suspen- 

 sion then produces upon it a tractive force through the instrumentality 

 of the rod, which has become inclined to the horizon. That traction 

 has a horizontal component, and ought, consequently, to draw forward 

 the center of the little mass, with a force proportionate in its intensifc r 

 to the speed of the vehicle. In falling back again, and after having 

 retrogressed a little behind the verticle, as soon as the obliquity of the 

 rod has become sufficient, the same traction will exert itself anew, and 

 the bob will re-commence the same oscillation forwards, under the in- 

 fluence of the two forces of traction and gravity. 



Without knowing precisely the law of these two combined actions, 

 we may nevertheless conjecture that the pendulum, under these cir- 

 cumstances, would take a deviation forwards, great or small in propor- 

 tion to the velocity of traction, and remaining sensibly constant if the 

 same speed were maintained. An experimental graduation might then 

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