172 ANIMAL PHYSIOLOGY. 
ducing the movement of a cylinder, which may be covered with 
smoked paper, or otherwise prepared to receive impressions 
made upon it by a point and capa- 
ble of being raised or lowered, and 
its movements regulated. The 
cylinder is ruled vertically into 
a certain number of spaces, so 
that, if its rate of revolution is 
known and is constant (very im- 
portant), the length of time of 
any event recorded on the sen- 
sitive surface may be accurately 
known. This whole apparatus 
may be considered a chrono- 
graph in a rough form. 
But a tuning-fork is the most 
reliable form of chronograph, 
provided it can be kept in con- 
stant action so long as required; 
and is provided with a recording 
apparatus that does not cause 
enough friction to interfere with 
: its vibrations. 
Fic. 164.—Original chronometer, devised b . G 
Thomas Young, for measuring panies Fig. 166 illustrates one ar- 
Piece fumeerecie’s The marker, or chrono 
line on cylinder, : uf graph, 
in the more: limited sense, is 
kept in automatic action by the fork interrupting the current 
from a battery at a certain definite rate answering to its own 
proper note. 
hun 
Fie. 165.—Myographic tracing, such as is obtained when the cylinder on which it is written 
does not revolve during the contraction of the muscle (after McKendrick). 
Marey’s chronograph, which is represented at h above, and 
in more detail below, in Fig. 167, consists of two electro-magnets 
armed with keepers, between which is the writer, which has a 
