28 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [2^] 



& Zambra, but, under the above conditions, a delay of a few minutes 

 is not of great importance. The movable indices are a fruitful source 

 of annoyance and vexatious delay. An index may, without any ai)i)ar- 

 ent cause, absolutely refuse to move in the tube ; coaxing with the mag- 

 net is followed by lightly tapping the 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 measures, none of which tend to improve 

 the general condition of the instrument. 



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

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

 ful handling. 



Most of the minor casualties to which the instrument is liable are ap- 

 parent to the eye and are readily set right. 



WATER DENSITIES. 



Hilgard's ocean salinometer (Plate XVIII) is used by the Commis- 

 sion for observing the density of sea- water. We found it difficult at 

 times to use this delicate instrument at sea, until we adopted the plan 

 t»f' setting it on a nicely adjusted swinging stand, which rendered it to 

 a great extent independent of the movements of the vessel. 



An excellent description of the apparatus is given by Professor J. E. 

 Hilgard in the Coast Survey Eeport for 1874, and reproduced in Sigsbee's 

 Deep-sea Sounding and Dred ging, 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 deter- 

 mination. 



The instruments employed for the jiurpose have been, almost without 

 exception, areometers of various forms. The differences of density as 

 arising from saltness are so small that it is necessary to have a very 

 sensitive instrument. As the density of ocean water at the tempera- 

 ture of COO Fahr. only varies between the limits of 1.024 and 1.029, it is 

 necessary, in order to determine differences to the hundredtli part, that 

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

 decimal i^lace. 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 assuaies dimensions which make it unfit for use on board 

 ship. 



With a view to the convefiient adaptation to practical use, this appa 

 ratus 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 thous- 

 andths 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 



