ON SPHYGMOGRAPHY. 65 
surrounding it, which has a diameter of about one-sixth of an inch. 
On its outer surface this ring is grooved and an endless screw is cut in 
this groove. : 
There is no brasswork attached to the pulse-spring corresponding 
to that in the older instrument. In its stead a long flat slip of brass, 
fixed about two-thirds the length of that spring from its attached end, 
extends onwards over the ivory pad. Just above this pad a steel screw 
about an inch long, with a milled-head at one end, is fixed at the other 
to the unattached extremity of the piece of brass just mentioned, by 
a hinge, and is so arranged that by means of the pulse-spring it is 
retained either at right angles to it, or lying along it, on the same 
principle that an ordinary knife-blade can be fixed half open or closed. 
This long screw, when at right angles or nearly so, to the spring, 
comes in contact with the endless screw cut in the brass ring surround- 
ing the arbor, and as they are of corresponding size, they bite and are 
retained in contact by the pulse-spring pressing on the fixed end of the 
screw, which is squared off in such a manner that it shall continue to 
press slightly backwards when in contact with the ring. It is evident 
from this construction that any up or down movement communicated 
to the pulse-pad, produces a corresponding rise or fall in the recording 
lever. When not in action the long serew can be thrown out of gear, Page 402. 
down on the spring, as seen in Fig. 2, and it has also a movement 
-which allows of its being turned round, which can be employed as a 
fine adjustment in regulating the height of the lever point. 
Tn this instrument the distance between the axis of rotation of the 
arbor and the part of the long serew which is in contact with the ring 
surrounding it, cannot vary, whatever the distance of the pulse-pad, 
and therefore no error is introdueed in that direction. Also the diffi- 
culty in disconnecting the spring from the axis, after the trace has 
been taken, is very slight; though it requires some little practice to do 
this, before removing the instrument from the wrist, as it always 
should be. 
This, the rackwork sphygmograph, will probably supersede the 
kmnife-edge one by degrees, for the tracings are more uniform, and in 
other respects quite as good as those obtained by the earlier instrument. 
For cardiograms it is not so advantageous. 
With regard to the best means of binding the sphygmograph on 
the arm, the original method adopted by Marey, of lacing it with a 
silk ribbon to the side-lappets, is as efficient as any. The wrist should 
be always bent a little backwards, and care should be taken that the 
pad presses on the lower end of the radial artery, not on the super- 
ficialis vole, as is apt to occur if itis fixed too far forwards. The pad 
introduced by Mr. Berkeley Hill enables the correct position of the 
hand to be maintained with facility, but it is scarcely necessary, as a 
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