282 Lieut. -Colonel C. B. Heald and Major W. S. Tucker 



record upon a smoked drum the movements of the table multiplied by 100. 

 His remarks on this table are interesting : — 



" With such a table, the heart beat causes not only longitudinal, but 

 also lateral movements. The latter have not yet been examined in 

 detail. In fact, it is necessary, in order to record the longitudinal move- 

 ments with accuracy, that the lateral movements should be prevented. 

 It is also necessary, not only that the person under examination should 

 lie absolutely still, but that he stop breathing during the time the heart 

 beats arc recorded. Herein, indeed, lies the chief difficulty of the investi- 

 gation, for while the total amplitude of the recoil movements is only a 

 tenth of a millimetre, and some of the features of the curve amount to 

 less than a tenth of this distance, respiration swings the body through a 

 distance of many millimetres. Lastly, in order to avoid, so far as 

 possible, errors from the table swinging back, after being moved out of 

 plumb by the recoil, a pendulum period many times longer than the 

 cardiac cycle was found necessary." 



This author's comments, even with these precautions, showed that at least 

 three great difficulties arose : — 



(1) Periodicity in the recording apparatus. 



(2) Errors due to respiratory movements. 



(3) Errors due to the records having to be taken when the breath is either 

 held, or the subject is blowing on a whistle. 



He considers that if these difficulties could be overcome, our knowledge of 

 the factors controlling the physical efficiency of the cardio-vascular system 

 would be improved. 



It appeared reasonable to think that the hot-wire microphone would give 

 faithful records without encountering any of the difficulties above referred to, 

 and, at the same time, provide a means of measurement and calibration. 



It was realised that if such measurements and calibration could be accom- 

 plished, the results would be of value in examinations for determining physical 

 efficiency, especially in the case of aeroplane pilots. 



The recording apparatus (fig. 1) may be divided into three parts : — 



(1) The microphones. 



(2) The galvanometer and timing device. 



(3) The photographic apparatus. 



It may be here stated that some of this apparatus has already been in use 

 in sound ranging of guns, but important modifications have been made in 

 order to cope with the special difficulties of the method. 



