272 MEMORIAL OF JOSEPH HENRY. 



In 1843, he read a communication to the Society, "On a new 

 method of determining the velocity of Projectiles:' 7 for this purpose 

 employing two screens of fine insulated wire each in circuit with a 

 galvanometer, and at determined near distances in the path of the 

 projectile; whereby the galvanic currents would be successively 

 interrupted at the instants of penetration. To record the interval, 

 each galvanometer needle is provided at one end with a marking 

 pen touching a horizontally revolving cylinder, which is divided by 

 longitudinal lines into 100 equal parts, and is driven by clock-work 

 at the rate of ten revolutions per second, giving therefore to the 

 interval of passage between two consecutive lines, the thousandth 

 part of a second. * Another still more ingenious method is sug- 

 gested, whereby the galvanometer may be dispensed with : each 

 circuit including an induction coil, one end of whose secondary 

 circuit is connected with the axis, and the other end placed very 

 nearly in contact with the surface of the graduated paper on the 

 revolving cylinder, so as to give the induction spark through the 

 paper at the instant of the interruption of the primary circuits by 

 the projectile passing through the wire screens. This is really a 

 much neater and more direct application of the electric interruption 

 than the employment of a galvanometer needle for making the 

 record, as it involves no material inertia. If desirable, the cylinder 

 may be made to have a very slow longitudinal movement by a screw, 

 so as to give a helical direction to the tracings; and different pairs 

 of screens similarly arranged at distant points in the path of the 

 projectile may be employed to determine the variations of velocity 

 in its flight, f 



Henry was always a watchful student of psychological and sub- 

 jective phenomena. Witnessing on one occasion the performance 

 of an athlete before a large assembly, he noticed with a curious 

 interest the "inductive" sympathy manifested by nearly every 

 spectator (himself included) in being swayed by a movement as of 



*It appears that WHKATSTONE devised his ingenious electromagnetic "chrono- 

 scope" in 1840; though he unfortunately published no account of it till 1845; or 

 two years after the publication by HENRY. And this was called out as a reclama- 

 tion, on the publication of a similar invention by L. BREGUET, of Paris, in January 

 of the same year. See "Supplement," NOTE Q. 



t Proceed. Am. Phil. Soc. May 30, 1843, vol. iii. pp. 16&-167. 



