166 SCIENCE PROGRESS. 



which had previously been adopted for other purposes 

 by our distinguished countryman, Thomas Young, but 

 which had been forgotten, so that Ludwig not only 

 founded the graphic method, but gave it back in a greatly 

 improved form both to physics and meteorology. 



When an artery is severed, as every one knows, the 

 blood comes out in jets ; a proof that the pressure within 

 the arteries exceeds the atmospheric pressure. The Rev. 

 Stephen Hales, Rector of Teddington, described in 1727 in 

 his Statical Essays how he introduced the end of a 

 long vertically placed tube, seven to eight feet in height, into 

 the femoral artery of a mare, and observed how high the 

 blood mounted in this tube. This it did to a height of 

 nearly seven feet, so that this represented the " blood 

 pressure". In 1828, Poiseuille, a French physician, 

 connected a blood-vessel of an animal with a U shaped 

 glass tube, or manometer, partly filled with mercury, one 

 end being connected to the artery by means of a lead tube 

 filled with carbonate of soda solution to prevent the 

 blood from coagulating. Thus any oscillations of the 

 mercury could be read off in the open limb of the tube. 

 This was a great advance on the method of Hales, but still 

 it was unsatisfactory. Ludwig, in 1847, modified this in- 

 strument by placing a light swimmer in the open limb of 

 the tube and causing the swimmer to record its movements 

 on a revolving surface moved by clockwork. By this 

 method a permanent graphic record was obtained, a curve 

 whose height was an expression of the blood pressure 

 within the artery, and its extent a measure of the time of 

 the several phases of the curve. For the first time the 

 pulsations of the living heart were recorded on paper. 



The story of the Kymograph is an interesting one. In 

 1846, while Ludwig was still in Marburg, he studied the 

 relation between the respiratory movements and the blood 

 pressure, and on his Kymographion he recorded the move- 

 ments of respiration and the variations of the blood pressure 

 simultaneously. As it were by inspiration the graphic 

 method was founded. For the first time on the same piece 

 of paper was recorded the respiratory movements of the 



