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The year is 1733

and the Enlightenment is in full force in England. Philosophers dream of a brighter age and scientists scramble to organize and establish their ranks amongst the new order of scientific academies. Cambridge scholar Stephen Hales calms an adolescent horse he's laid down on its side. He looks to his assistant and gives him the go-ahead. Complying with Stephen's request, the assistant proceeds to connect a 9 foot glass tube to a bypass Stephen had inserted into the crural artery of the horse's thigh. The connection had been made! The assistant points the glass tube vertical towards the sky. Stephen releases a tie. They both watch in anticipation as the warm blood of the horse's artery enters and climbs the walls of the glass tube. The blood quickly rises reaching a height of 8 feet 3 inches above the horse. Suddenly the blood level starts rising and falling about 3 inches in a continuous periodic nature that appears to be in synchrony with each of the horse's heart beats. Blood pressure is discovered.

 Stephen Hales

Fast forward. Paris 1828

Unsatisfied with the crudeness of Stephen Hales' approach, Jean Léonard Marie Poiseuille invents the mercury manometer, a method to measure blood pressure, and is commended with the gold medal of the Royal Academy of Medicine.

 Poiseuille's mercury manometer. 

Berlin 1847

Carl Ludwig finds a need to better quantify and record the measurements produced by Poiseuille's mercury manometer. He invents the kymograph, using a floating pen attached to Poiseuille's mercury manometer, and graphs the periodic nature of blood pressure for the first time. 

 

 The first graphs of blood pressure are produced with the kymograph, invented by Carl Ludwig.

Tübingen, Germany 1854

Karl von Vierordt proposes a method to measure blood pressure indirectly by measuring the counter-forces against an artery needed to occlude, or stop, its flow of blood, marking a major advancement in physicians ability to measure blood pressure non-invasively. The sphygmograph was born.

 Vierordt's sphygmograph. The pad is applied over the radial artery. Weights are placed in the large and small cups as a pulse wave is traced out.
 

The discovery of blood pressure was more important than the discovery of blood.
— Johannes Müller