CIRCULATION. 



131 



lunar, may also be ascertained. The practical difficulties in the way of 

 applying- this method to the ventricle and auricle are much greater than to the 

 ventricle and artery. By another application of the principle just described, a 

 "differential manometer" has been devised for the purpose of registering as a 

 single curve the successive differences, from moment to moment, between the 

 ventricular and auricular pressures, or the ventricular and arterial pressures 

 (see Fig. 25). To this end, two elastic manometers are fastened immovably 

 together, and their two elastic disks, instead of bearing upon separate levers, 

 are made to bear upon a single one, which has its fulcrum between the disks, 

 and is a lever not of the third order, but of the first, like a common balance. 



Fig. 25.— Diagram of the differential manometer: A, artery: V, ventricle; Z>, drum of kymograph, 

 revolving in the direction of the arrow, and covered with smoked paper; L, recording lever in contact 

 with the revolving drum ; 8, a spring by which the movement of the lever worked by the disks is trans- 

 mitted to the recording lever. (The working details of the instrument are suppressed or altered for the 

 sake of clearness.) 



As the lever or beam of the balance turns from the horizontal as soon as the 

 scales are pressed upon by unequal weights, so the lever of the differential 

 manometer turns as soon as the disks are unequally affected by the pressures 

 within the ventricle and the auricle, or the ventricle and the artery. As, how- 

 ever, the pressures upon the scales are from above, while those upon the disks 

 are from below, the disk which tends to "kick the beam" is the one acted 

 upon by the greater pressure, instead of by the less, as in the case of the scales. 

 The manometric lever marks its oscillations as a curve upon the kymograph 

 by the help of a second or" writing lever" connected with it. The persistence 

 of exactly equal pressures, no matter what their absolute value, in the two 

 manometers would cause a horizontal line to be drawn by the writing lever. 

 This would serve as a base-line. The differential manometer is ;i valuable 

 instrument, although it is evident that where such minute differences of space 

 and time are recorded as a curve by such complicated mechanisms, the sources 

 of error must be numerous and difficult to avoid. 1 



The methods which proceed by the measurement of differences of pressure 

 may sometimes be controlled, or even replaced, by an easier method, as follows: 

 If two manometers simultaneously record on the same kymograph thepressure- 

 1 K. Hurthle: Pfluger'a Arehiv fur die gesammie Phyiriologie, 1891, Bd. 49, S. 45. 



