1876.] 



the Piezometer in Deep- Sea Sounding. 



163 



instead of water. The portion of the stem in which the index moves is 

 filled with water, and, as in the other, the open end dips into a cup of 

 mercury. We have thus an instrument filled with a very large quantity 

 of mercury and a very small quantity of water ; and after immersion the 

 position of the index shows the apparent volume assumed by this mix- 

 ture under the combined influence of temperature and pressure. As far 

 as the effects of temperature are concerned, the amount of water in the 

 instrument is almost wholly negligible ; but v/hen the effect of pressure is 

 considered, the apparent compressibility of mercury is so small, being 

 little more than one fiftieth of that of water, that the pressure of even so 

 small a quantity of water as can be contained in the graduated tube in- 

 creases very materially the amount of contraction produced by pressure. 

 The instrument, which has been in use since the beginning of November 

 last year, and which is designated XVII. a, contains 2,bQ-Ql grammes of 

 mercury in the bulb and stem immediately above it; the volume of the 

 part of the stem filled with water is 0-1935 c. c. The apparent contrac- 

 tion of this mass of mercury and water is 0*000581 cubic centimetres per 

 100 fathoms, and 0-0025 c. c. per degree respectively. A fall therefore 

 of one degree in temperature produces the same effect as an increase of 

 pressure equal to 430 fathoms of sea-water. Hence (and this forms the 

 important peculiarity of the instrument) as long as the temperature of 

 the sea does not increase with the depth at a greater rate than 1° C. per 

 430 fathoms, the instrument w411 record the temperature correctly. The 

 ratio subsisting between the amount of temperature and the column of 

 water, which produce the same effect on the apparent volume, is a con- 

 stant for every instrument ; in our one it is ^^y. By altering only very 

 slightly the amount of water, the sensibility to pressure is greatl}^ in- 

 creased or diminished, while that to temperature remains practically 

 unchanged. As the instrument XYII. a was intended principally for 

 bottom-waters, the above ratio (7^3-0) was considered sufficient, and it has 

 proved practically useful. It must be remembered that the greater the 

 value of this ratio is made, the greater is the error introduced into the 

 determination of the temperature by any inaccuracy in the measurement 

 of the depth. 



By attaching a combination of one, or better of two, of each of these 

 instruments close to the weight at the end of the sounding-line, the 

 depth of the sea and the temperature of the water at the bottom at any 

 locality can be accurately determined, provided that sufficient evidence is 

 afforded, either by the presence of a sample of bottom in the sounding- 

 tube, or by the rate at which the line runs out, that the instruments have 

 been at the bottom ; otherwise the depth which they have attained and 

 the temperature at that depth will be correctly given. Eor this purpose 

 it is necessary first to let the line run out until, from observations on its 

 velocity, it is evident that the weight has reached the bottom ; the length 



