There are several instruments which are in common use for the purpose of recording blood pressure in man. Historically, the determination of blood pressure for man began with the attempt of K. Vierordt in 1855 to measure the blood pressure by placing weights on the radial pulse until this was obliterated. The first useful instrument, however, was devised by Marcy in 1876. He placed the hand in a closed vessel containing water connected by tubing with a bottle for raising the pressure and by another tube with a tambour and lever for recording the size of the pulse waves. He maintained that when pressure on the hand was made, the point where oscillations of the lever ceased was the maximal pressure, the point where the oscillations of the recording lever was largest, was the minimal pressure. This pioneer work was practically forgotten for twenty-five years. It was not until 1887 that V. Basch devised an instrument which was used to some extent. This instrument recorded only maximum pressure. It consisted of a small rubber bulb filled with water communicating with a mercury manometer. The bulb was pressed on the radial artery until the pulse below it was obliterated and the pressure then read off on the column of mercury. V. Basch later substituted a spring manometer for the mercury column. Potain modified the apparatus by using air in the bulb with an aneroid barometer for recording the pressure. These instruments are necessarily grossly inaccurate. Moreover, they do not record the diastolic pressure. In 1896 and 1897 further attempts were made to record blood pressure by the introduction of a flat rubber bag encased in some nonyielding material, which was placed around the upper arm. Riva-Rocci used silk, while Hill and Barnard used leather. The latter used a bulb or Davidson syringe to force air into the cuff around the arm and palpated the radial artery at the wrist, noting the point of return of the pulse after compression of the upper arm, and reading the pressure on a column of mercury in a tube. Except that the width of the cuff has been increased from 5 cm. to 12 cm., this is the general principle upon which all the blood pressure instruments now in use are based. Most of the apparatuses make use of a column of mercury in a U-tube to record the millimeters of pressure. As the mercury is depressed in one arm to the same extent as it is raised in the other arm the scale where readings are made is .5 cm. and the divisions represent 2 mm. of mercury but are actually 1 mm. apart.