170 



THE GRAPHIC METHOD, 



BY the graphic method is meant the process by 

 which curves or tracings are obtained, which represent 

 various phenomena. Thus, a chart on which is traced 

 out daily the course a vessel has taken in crossing 

 from Europe to America is a graphic representation 

 of its voyage, the lines drawn from day to day repre- 

 senting not only the course of the vessel, but the 

 distance accomplished since the day previous, and con- 

 sequently the speed of the ship. Similarly a fever 

 chart, on which is marked daily the temperature of a 

 patient, each degree or fraction of a degree gained or 

 lost above the normal being represented by a mark at 

 a definite distance above or below the normal line, 

 and each successive day being indicated by a given 

 space across the chart, a fever chart is a graphic 

 representation of the course and variation of the 

 fever. Now this method is applied in many ways in 

 physiology and medicine, to obtain a record of time 

 of movement of heart, pulse, muscle, or chest, to obtain 

 a record of blood pressure, and so on. The means of 

 recording time will first be considered. 



CHAPTER XVI. 



THE MEASUREMENT OF SMALL INTERVALS OP TIME. 



THE idea which has rendered possible great 

 advances in graphic registration, and especially that 

 of time, was suggested by Thomas Young in 1807. 



