In all the mercury instruments the diameter of the tube is 2 mm. One would suppose that there would be noticeable differences in the readings of the different mercury instruments depending upon the amount of mercury used in the tube. By actual weight there is from 35 to 45 gms. of mercury in the several instruments. After many trials, no noticeable differences in blood pressure readings can be made out between a column weighing 35 gm. and one weighing 45 gm. There is, however, the inertia of the mercury to be overcome, friction between the tube and the mercury, and vapor tension. The mercury is therefore not as sensitive to rapid changes of pressure in the cuff as a lighter fluid would be. The mercury must be clean and the tube dry so that there is no more friction than what is inherent between the mercury and glass. In making readings on a rapid pulse the oscillations of the mercury column are apt to be irregular or to cease now and then, due to the fact that the downward oscillation coincides with a pulse wave, or an upward oscillation receives the impact of two pulse waves transmitted through the cuff. Instruments have been devised to obviate this difficulty, but they have not come into favor. They are usually too complicated and at present can not be recommended. [Illustration: Fig. 18.--Rogers' "Tycos" dial sphygmomanometer.] An instrument devised by Dr. Rogers (the "Tycos") has met with considerable popularity. (Fig. 18.) This is not an instrument which operates with a spring and lever. The instrument is composed essentially of two metal discs carefully ground and attached at their circumferences to the metal casing below the dial. There is an air chamber between these discs through the center of which air is forced by the syringe bulb. When air is forced into the space between these two discs, they are forced apart to a very slight extent, with the highest pressures only 2-3 mm. of bulging occurs. From data gathered after extensive use for five years these discs were not found to have sprung. A lever attached to a cog which in turn is attached to the dial needle magnifies to an enormous extent the slightest expansion of the discs. Every dial is handmade and every division is actually determined by using a U. S. government mercury manometer of standard type. No two dials therefore are alike in the spacing of the divisions of the scale but every one is calibrated as an individual instrument. There is no doubt in the author's mind that for the general practitioner the instrument has some advantages over the mercury instruments. It reveals the slightest irregularity in force of the heart beat. The oscillation of the dial needle is more accurately followed by the eye than is that of the column of mercury. The needle passes directly over the divisions of the scale, while with usual mercury instruments the scale is an appreciable distance (sometimes .5 cm.) from the column of mercury at the side. (Fig.