We have here on the table two or three kinds of recording apparatus. The one in front of me, which looks rather a large instrument is what is called an “endless paper apparatus;” by using which one can go on making an observation for a very long time without altering the apparatus at all. On one portion there is a roll of paper rolled by machinery, some hundreds of yards long. The whole object of the machine is to unroll this paper, moving it along at a regular rate, and passing it over these brass cylinders, and then as the paper is passing, any movement in the animal body which we wish to record is marked on the paper as it passes along. You must have noticed in the shops of mathematical instrument makers, clocks provided with a rotating drum, on which a piece of paper is carried round very slowly according to the movement of certain wheels of the clock, and you have also noticed, marking upon such a drum, a pencil which moves up and down, and this pencil is connected perhaps with a barometer. The apparatus here is founded on precisely the same principles. It may be moved either by a gas engine, a steam engine, or by clockwork, and the rate is capable of being varied by a screw underneath. A curve may be traced on the moving paper by means of a bent U tube or manometer, filled with mercury to a certain height, and open at each end. At the top of the mercury in one of the legs of the tube, lies a float, which is connected to a little pen and follows the movements of the mercury. The pen is wetted with ink so as to write on the white paper and trace the movement upon it. If we wish to take 282the pulse in the arteries we have only to connect the other end of the tube by means of an india-rubber pipe with an artery, and then any movement in the artery will cause the mercury to rise and fall. In this way many of the principal discoveries, with regard to the phenomena exhibited in the circulation of the blood, have been made. This apparatus we owe to Professor Ludwig, and it is the ordinary apparatus used in physiological laboratories for obtaining a trace of the movement of the blood and the variations in the pressure which is found within the system of blood-vessels.