66 



THE POPULAR SCIEXCE MONTHLY. 



Fig. 5. 



irrx. 



I 



P 





the observer in bringing about a catastrophe whicli shall leave its 

 mark on the condition of the instrument. 



All the self-resfisterinfj thermometers are liable to error from the 

 effects of pressure, which may amount to five or six hundred atmos- 

 pheres on the outside of tlie instrument, v/hile inside it -is never 

 greater than was that of the atmosphere Avhen the tube was sealed up. 

 Attempts to obviate them have been made by placing the thermome- 

 ters or their bulbs in protecting inclosures, and by the device of leav- 

 ing the instrument open at one end. This was adopted by Aime 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 construct- 

 ed a mercurial thermometer, or piezometer (Fig. 5) on the 

 same principle, but his object in admitting the water-press- 

 ure to the inside of the instrument was to utilize it in shift- 

 ing the scale of the thermometer as the depths changed. 

 The thing registered in such instruments is always the ap- 

 parent volume of the liquid, and this varies with the tem- 

 perature and the pressure. Hence the indications will rep- 

 resent 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 in- 

 creased, 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. Ko one instrument, how- 

 ever, fulfills all the conditions required of a perfect deep-sea ther- 

 mometer, 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 extrem- 

 ity, 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 





