264 Mr. F. W. Lanchester on 



axis. The only theoretical disadvantage of this arrangement 

 is that a small time error is introduced, but provided the 

 pencil arm be made reasonably long this error is infinitesimal. 



In order that the accuracy of the instrument shall not be 

 interfered with, the pendulum should be made as short as 

 possible. It is assumed in the theory of the instrument, that 

 the motion of the pendulum-bob is substantially that of the 

 rest of the vehicle, and consequently its motion of swing- 

 should be negligible in comparison with the motion of the 

 vehicle ; also its time of oscillation should be small in com- 

 parison with the time of change of acceleration that it is 

 desired to accurately record. This point was not overlooked 

 in the original model, in which the length of pendulum was 

 3 inches only ; in the present model this has been reduced 

 to 1J in., beyond which it is doubtful whether it is desirable 

 to go. 



The dash-pot, which is an absolute necessity for working 

 on road vehicles, in the original instrument took the form of an 

 oil-well of large dimensions in which dipped a vane or paddle 

 of considerable area. On the present model the dash-pot is 

 a simple cylinder with spherical piston making an approximate 

 fit, the cylinder is filled with viscous oil and is adjusted to 

 make the pendulum as nearly as possible " dead- beat/' 



Considered purely as an acceleration meter, a correction 

 would be necessary for any change of gradient; and if it were 

 required, for instance, to integrate the diagrams to obtain 

 velocity curves, either the precaution would have to be taken 

 to work on level roads, or means found of making the neces- 

 sary corrections. It is a point of great interest and importance 

 that for the purposes for which this instrument is principally 

 required, starting and stopping efforts, frictional and other 

 resistance, &c, the readings are unaffected by gradient and 

 no corrections are required. Put tersely, it may be said that 

 the acceleration produced by the gravity component exactly 

 compensates or neutralizes the direct effect of the gradient. 



This point is one that it is quite easy to make clear. Let 

 fig. 5 represent a car standing with brake applied on a 

 gradient whose angle of slope is /3, then the accelero meter 

 will record an amount equal to <j tan (3 (provided that it has 

 previously been brought to zero on the level): now this will 

 be a correct measure of the brake and frictional effect 

 employed in holding the car up. Now let us suppose the 

 brake removed, then the true acceleration (neglecting friction) 

 will rise from zero to g tan /3, which will bring the accelero- 

 meter reading exactly to zero, which under these circum- 

 stances is the precise measure of the applied tractive forces. 



