268 The Pendulum Accelerometer. 



investigating this jerk in 1888 that the idea o£ the pendulum 

 accelerometer occurred to the writer. At that time it was 

 currently supposed that the jerk was the effect of the recoil 

 of the buffer springs after stopping; whereas a very little 

 consideration shows that it is in reality sudden change of 

 acceleration that we recognize physiologically as "jerk/' that 

 is df/dt, and not change in the direction of motion. It sug- 

 gests itself in fact that the term "jerk " might well be given 

 a scientific meaning and be defined as d 3 s/dt*. 



The value of df/dt at the moment of stoppage in the case 

 of a rigid body sliding on a rigid plane surface would be 

 infinite. In actual fact, in the case of a train brought to rest 

 by its brakes the value of df/dt is so high that the correct 

 form of the curve at the moment of stopping is beyond the 

 power of the instrument to properly record. In all the 

 diagrams the lag of the pendulum will be noticed, and. in 

 most cases the line shows that the " dash-pot " has been 

 insufficient and the pencil has overshot the mark. The 

 peculiar form found on diagram 3 fig. 6 is due to a partial 

 vacuum formed in the dash-pot on the sudden release of the 

 pendulum from its extreme position. It is probable that the 

 correct form of the drop would be sensibly perpendicular to 

 the datum-line. 



It should be more widely recognized that a great part of 

 the art of properly braking a train, or in fact any other 

 vehicle, consists in taking the brake nearly off just before 

 stopping. It is not easy to obtain a train diagram in which 

 this has been done, but diagram 1 in fig. 7 is a good example 

 taken on a car in which the driver was paying special attention 

 to the point. 



Many of the diagrams, figs. 6 and 7, are particularly 

 interesting to the motor-car engineer : the loss of starting 

 effort area (which is equivalent to velocity) during gear 

 change, the exalted starting effort obtained whilst the energy 

 of the flywheel is being drawn upon, the curve of falling 

 effort (largely due to increasing wind resistance), — all these 

 are points that repay the most careful study. 



Other applications I anticipate for the pendulum accelero- 

 meter are its employment in the determination of the lateral 

 (centrifugal) forces and coefficient of friction when rounding 

 corners, the determination of wind resistance at different 

 speeds and other resistances to traction, the determination of 

 the thrusts and resistances of launches &c. ; but these and the 

 further discussion of the engineering aspect of the subject 

 does not come within the purview of the present paper. 



May 11th, 1905. 



