98 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



the other terminal of T, and ploughed a clean furrow in the lead as the 

 weight, in falling, carried the lead by. In this way sure contact across 

 the gap could be made for a time interval, which could be varied be- 

 tween limits, the length of which could be computed with some accu- 

 racy. The agreement between the results obtained by this device and 

 those calculated from the electrical constants was always so close as to 

 lie within the uncertainty of the zero of the galvanometer. 



Using the apparatus just described, a large number of trials were 

 made by about twenty different persons to determine how short a 



TABLE I. 



Observer. 



B 



C 



D 



E 



F 



G 



11 



I 



J 



K 



Average time of 

 contact of key 



as obtained from 

 all the trials. 



0.0329 

 0.0278 

 0.0308 

 0.0223 

 0383 

 O.OGOO 

 0.0239 

 0.0871 

 0.0210 

 0.0239 



Average time of 



contact of key 



as obtained from 



all the trials. 



0.0203 

 0.0303 

 O.OIGG 

 0.0175 

 0.0107 

 0.0277 

 0.0416 

 0.0318 

 0.0350 



Average time of 

 contact of key a.s 

 obtained from tlie 



shorter half of 

 the observations. 



0.0130 

 0.0257 

 0.0095 

 0.0112 

 0.0050 

 0.0112 

 0.0362 

 0.0284 

 0.0280 



contact each could surely make with a telegraph key or with a thim- 

 ble on a plate. If a telegraph key is furnished with a very stiff spring 

 it is possible for any one, after a little practice, to make short contacts, 

 not more than a thousandth of a second long, by striking the key with 

 the hand ; the observations recorded below were all made with a key 

 which had no spring, and which had to be lifted, after the contact, by 

 hand. It generally appeared that in the course of a long series of 

 trials a person made two or three contacts of abnormal length (some- 

 times five or ten times as long as the mean of the others), but it did 

 not seem well to leave them out of account ; I have, therefore, given 



