[75] U. S. FISH COMMISSION STEAMER ALBATROSS. 77 



frame in the hand or swinging it rapidly about the head ; and if this 

 fails more vigorous tapping is apt to follow with various active meas- 

 ures, none of which tend to improve the general condition of the instru- 

 ment. 



The indices are also liable to move if the instrument is subjected to 

 rough treatment, although this is not of frequent occurrence with care- 

 ful handling. Most of the minor casualties to which the instrument is 

 liable are apparent to the eye and are readily adjusted. 



WATER DENSITIES. 



Hilgard's ocean salinometer (Plate XLI) is used on board of the Al- 

 .batross for observing the density of sea- water. 



An excellent description of the apparatus is given by Prof. J. E. Hil- 

 gard in the United States Coast Survey Beport for 1874, and reproduced 

 in Sigsbee's Deep-sea Sounding and Dredging, as follows: 



"The density of sea-water in different latitudes and at different depths 

 is an element of so great importance in the study of ocean physics as 

 to have caused a great deal of attention to be paid lately to its determi- 

 nation. The instruments employed for the purpose have been, almost 

 without exception, areometers of various forms. The differences of 

 density as arisiug from saltness are so small that it is necessary to have 

 a very sensitive instrument. As the density of ocean water at the tem- 

 perature of 60° Fahr. only varies between the limits 1.024 and 1.029, it 

 is necessary, in order to determine differences to the hundredth part, that 

 we should be able to observe accurately the half of a unit in the fourth 

 decimal place. This gives a great extension to the scale, and involves 

 the use of a series of floats if the scale starts from fresh water, or else 

 the instrument assumes dimensions which make it unfit for use on board 

 ship. With a view to the convenient adaptation to practical use this 

 apparatus has been devised for the Coast Survey by Assistant Hilgard. 



"The instrument consists of a single float about 9 inches in length. 

 The scale extends from 1.020 to 1.031, in order to give sufficient range 

 for the effect of temperature. Each unit in the third place, or thou- 

 sandths of the density of fresh water, is represented by a length of 0.3 

 of an inch, which is subdivided into five parts, admitting of an accurate 

 reading of a unit in the fourth place of decimals by estimation. 



"The float is accompanied by a copper case, with a thermometer in- 

 serted within the cavity, which is glazed in front. In use the case is 

 nearly filled with water, so as to overflow when the float is inserted, 

 the reading then being taken with ease at the top of the liquid. 



" For convenience and security, two such floats and a case are packed 

 together in a suitable case, and a supply of floats and thermometers 

 securely packed in sawdust is kept on hand to replace the broken ones. 



"The following table has been derived from the observations of the 



