=The Thread Recorder.=—In this instrument an intermittent record is secured in ink, possessing the advantages of visibility during the period over which readings are taken, and of permanence without subsequent treatment of the chart. The principle is shown in fig. 21, where A is a boom terminating in a V-shaped piece of ivory, and attached to the galvanometer suspension B. By means of a cam E, rotated by clockwork, a bar D is made to descend at stated intervals, pressing the end of A on to an inked thread G, and causing the thread to touch a paper wound round the drum C. This drum rotates on its axis once in 25 hours by the action of internal clockwork. The continued rotation of the cam E alternately raises and depresses the boom A, leaving it free for a sufficient time to enable it to attain the position it would occupy if the mechanism were absent. The thread G is passed over pulleys, and is wound round through an ink-well, so that the portion opposite A is always moist. With the bar D descending every two minutes, the successive dots form a nearly continuous line. The paper on C is divided horizontally into temperatures, and vertically into time units, so that the temperature existing at any given time may readily be ascertained. The front of the bar D, or a separate strip parallel to it, is divided so as to enable temperatures to be read without reference to the chart. The actual instrument is shown in fig. 22. When several simultaneous records are required, the drum C is extended, and other galvanometers introduced, to which the separate pyrometers are connected. Several records can be taken on one chart by introducing a clockwork mechanism to couple each pyrometer in turn to the one galvanometer. [Illustration: FIG. 22.—THREAD RECORDER.] [Illustration: FIG. 23.—SIEMENS’ RECORDER.] =The Siemens Recorder.=—In this instrument (fig. 23) the boom from the galvanometer terminates in a knife-edge, and moves over a thin horizontal rail, the top of which is rounded. Between the rail and the boom are placed an inking ribbon and a paper chart, which is moved forward by clockwork. A chopper-bar, also actuated by clockwork, descends at about half-minute intervals, and depresses the end of the galvanometer boom, thus producing a small dot on the chart. The paper is 12 cms. wide and 40 yards long; it is divided into time and temperature units, and moves forward at the rate of 2 cms. per hour. Levelling screws are fixed to the base of the recorder. [Illustration: FIG. 24.—FOSTER’S RECORDER.]