236 LECTURES TO SCIENCE TEACHERS. 



other instruments of the same kind, is derived from the 

 Greek word /cv/ua, which signifies a wave. It is so called 

 because it is used to record the wave-like variations of 

 arterial pressure, just as other instruments, of which there 

 are beautiful examples in this collection, are used by the 

 physicist to record the variations of the surface of the 

 ocean. Just as in the ocean two principal orders of varia- 

 tion have to be recorded, viz., tides, and waves in the 

 ordinary sense, so here there are slow variations of con- 

 siderable duration which recur at relatively long intervals, 

 and others which are by comparison momentary, but of 

 great frequency. Of the former, the mercurial column 

 gives a direct translation, so that the record of its ascents 

 and descents is equivalent to a direct record of the expan- 

 sions and contractions of the artery itself. But as regards 

 the more frequent ones it is not so, for this reason, that any 

 sudden impulse imparted to the column of mercury in a 

 manometer gives rise to a series of consecutive oscillations, 

 of which the duration and character have nothing to do 

 with that of the motion originally imparted, but depend 

 on the form of the instrument. They are therefore called 

 instrumental. 



This being the case, the mercurial kymograph has only 

 a limited application, as an instrument for recording the 

 actual movements of an artery. It is perfectly adapted 

 for the purpose of measuring and recording the mean 

 arterial pressure, and such variations as are of relatively 

 slow progress, but it is incapable of translating to us those 

 rapid changes which occur in the arterial pressure between 

 one stroke of the heart and another changes of which we 

 estimate the duration not in minutes, but in fractions of 

 seconds. 



That this defect is an important one is patent from the 

 following consideration : Every one is aware that the 

 quality of the pulse is matter of great practical interest. 

 Now, just as the physicist knows precisely how to account 

 for the difference of quality of different musical sounds, 

 by their physical characters, and is able to show that each 

 timbre may be represented graphically by a curve of which 

 the characters can be determined beforehand with mathe- 

 matical accuracy, so that, by the use of methods of 

 measurement, he acquires a power of discriminating 



