Six’s Self-registering Thermometer consists of a long tubular bulb, united to a smaller tube more than twice its length, and bent twice, like a syphon, so that the larger tube is in the centre, while the smaller one terminates at the top, on the right hand, in a pear-shaped bulb, as shown in the cut (Fig. 14). This bulb, and the tube in connection with it, are partly filled with spirit; the long central bulb and its connecting tube are completely filled, while the lower portion of the syphon is filled with mercury. A steel index, prevented from falling by a hair tied round it, to act as a spring, moves in the spirit in each of the side tubes. The scale on the left hand has the zero at the top, and that on the right at the bottom. When setting the instrument, the indices are brought into contact with the mercury by passing a small magnet down the outside of each tube. Then, should a rise of temperature take place, the spirit in the central bulb expands, forcing down the mercury in the left hand tube and causing it to rise in the right, and _vice versa_ for a diminution of temperature. It should be always used and carried upright, and the indices should be drawn _gently_ down by the magnet into contact with the mercury; and, when a reading is taken, the ends of the indices nearest the mercury indicate the maximum and minimum temperatures which have been attained during the stated hours of observation. [Illustration: 14. Six’s Thermometer. Scale about 1/7.] Six’s form of thermometer has been extensively used for ascertaining deep sea temperatures. [Illustration: 15. Deep Sea Maximum and Minimum Registering Thermometer. Scale about 1/5.]