PHILOSOPHICAL SOCIETY OF WASHINGTON. 283 



one second and two-tenths from the mean estimate of relative 

 longitude arrived at by other methods.* 



Ill 1840, Henry gave an account of "electricity obtained from 

 a small ball partly filled with water, and heated by a lamp."-|- 



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

 method of determining the velocity of Projectiles :" for this pur- 

 pose 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 re- 

 cord the interval, each galvanometer needle is provided at one 

 end with a marking pen touching a horizontally revolving cylin- 

 der, 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 suggested, whereby the galvano- 

 meter may be dispensed with: each circuit including an induc- 

 tion coil, one. end of whose secondaiy 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 pass- 

 ing through the wire screens. This is really a much neater and 

 more direct application of the electric interruption than the em- 

 ployment of a galvanometer needle for making the record. If 

 desirable, the cylinder may be made to have a very slow longi- 

 tudinal 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.§ 



Henry was always a Avatchful student of psychological and 

 subjective phenomena. Witnessing on one occasion the perform- 

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

 curious interest the "inductive" sympathy manifested by nearly 



* Proceed. Am. Phil. Soc. Deo. 20, 1839, vol. i, pp. 162, 1G3. "This 

 appears to have been ttie first actual determination of a ditrVreiice of lon- 

 gitude by meteoric obsej'vations." {L. E. D. Phil. Mag. 1841, vol. xix. 

 p. 553.) Seveial years later (in 183S) similar ineteorio observations were 

 made between Altona and Bre.siau ; and also between Rome and Naples. 



t Proceed. Am. Phil. Soc. Dec. 18, 1840, vol. i. p. 323. 



X It appears tliat Wneatstone devised his ingenious electro-mngnetic 

 " chronoscope" in 1S40; though he unfortunately published no account 

 of it till 184.') ; or two years after the publication by Henry. And this 

 was called out as a lechimation. on the publication of a similar invention 

 by L. Rretruet, of Pnris, in January of the same year. 



§ Proceed. Am. Phil. S^c. May 30, 1843, vol. iii. pp. 165-167- 



57 



