The most accurate mercury manometer is that of Erlanger. (Fig. 15.) The instrument is bulky and is not practicable for the physician in practice. The principle is that used by Riva-Rocci. There is an extra T-tube introduced between the manometer and air bulb connecting with a rubber bulb in a glass chamber. The oscillations of this are communicated to a Marey tambour and recorded on smoked paper revolving on a drum. There is a complicated valve which enables the operator to reduce the pressure with varying degrees of slowness. The mercury is placed in a U-tube with a scale alongside it. The instrument is expensive and not as easy to manipulate as its advocates would have us believe. Hirschfelder has added to the usefulness (as well as to the complexity) of the Erlanger instrument, by placing two recording tambours for the simultaneous registering of the carotid and venous pulses. In spite of its complexity and necessary bulkiness, very valuable data are obtained concerning the auricular contractions. [Illustration: Fig. 15.--The Erlanger sphygmomanometer with the Hirschfelder attachments by means of which simultaneous tracings can be obtained from the brachial, carotid, and venous pulses.] One of the best of the mercury instruments is the Brown sphygmomanometer. In this (Fig. 16) the mercury is in a closed, all-glass tube so that it can not spill under any sort of manipulation. It is in this sense "fool-proof." The cuff, however, is poorly constructed. It is too short and there are strings to tie it around the arm. I have found that this causes undue pressure in a narrow circle and renders the reading inaccurate. In the clinic we use this mercury instrument with a long cuff like that provided by the Tycos instrument. [Illustration: Fig. 16.--Desk model Baumanometer.] The Faught instrument (Fig. 17) is larger than the Brown, but is less easily broken and is not too cumbersome to carry around. The substitution of a metal air pump for the rubber makes the apparatus more durable. [Illustration: Fig. 17.--The Faught blood pressure instrument. An excellent instrument which is quite easily carried about and is not easily broken.] The v. Recklinghausen instrument is not employed to any extent in this country. It is both expensive and cumbersome, and has no advantages over the other instruments. Several other instruments have been devised and new ones are constantly being added to the already large list. With those employing mercury the principle is the same. The aim is to make an instrument which is easily carried, durable, and accurate.