Page:Popular Science Monthly Volume 19.djvu/76

66 the observer in bringing about a catastrophe which shall leave its mark on the condition of the instrument.

All the self-registering thermometers are liable to error from the effects of pressure, which may amount to five or six hundred atmospheres on the outside of the instrument, while inside it is never greater than was that of the atmosphere when the tube was sealed up. Attempts to obviate them have been made by placing the thermometers or their bulbs in protecting inclosures, and by the device of leaving the instrument open at one end. This was adopted by Aimé in some of his experiments, when the effect of pressure on the apparent volume of the liquid was determined independently, and a correction applied accordingly. The author has devised and constructed a mercurial thermometer, or piezometer (Fig. 5) on the same principle, but his object in admitting the water-pressure to the inside of the instrument was to utilize it in shifting the scale of the thermometer as the depths changed. The thing registered in such instruments is always the apparent volume of the liquid, and this varies with the temperature and the pressure. Hence the indications will represent the sum of the effects of the change of temperature and of pressure. If from any independent source we know either of these, we can determine the other. In a sea of uniform temperature throughout its depth, the apparent volume of the liquid would diminish as the pressure increased, and, if the temperature increased with the depth, the apparent volume of the liquid would diminish at a slower rate; but it would be always possible to determine the true temperature as long as it did not increase at so great a rate as to dilate the liquid more than it was compressed by the increasing pressure. For the investigation of seas such as the Mediterranean, this form of instrument is most valuable. No one instrument, however, fulfills all the conditions required of a perfect deep-sea thermometer, and the investigator must use his judgment in selecting the one or more best suited to his particular purpose.

The water from the bottom is usually collected in the so-called "slip" water-bottle. Water from intermediate depths is obtained in an instrument represented in section in Fig. 6. It consists of a cylinder, A, terminated at both ends by similar stopcocks, B, B, which are connected by the rod C. This rod carries, near its upper extremity, a piece of stout sheet-brass, D, ten centimetres long by fifteen broad, soldered to the casting E, which is movable about the axis e.

When intermediate water is to be obtained, the water-bottle is firmly attached to the sounding-line, which carries at its end usually a fifty-six pound or one hundred-weight lead; the stopcocks are then opened, giving them, with the rod C, the position represented in the figure. During the passage of the bottle downward, the water