596 FISHERIES OF THE UNITED STATES. [84] 



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

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

 venience and security two such floats and the can are packed together 

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

 packed in saw-dust, is kept on hand to replace the broken ones." 



Hilgard's Optical Densimeter, for determining the density of sea water by 



means of a prism. 



Prof. J. E. Hilgard, Superintendent United States Coast Survey. 



a # # # "When we get away from local conditions and inquire into the 

 general regimen of the ocean, affected in part by the fresh water out- 

 flow from the continents, but mainly by the general thermal circulation, 

 it becomes important to measure the differences of density with the 

 greatest precision that can practically be obtained. * * * The method 

 of ascertaining the density with hydrometers does not permit of great 

 precision on shipboard, because the float partakes of the movements of 

 the vessel, and oscillates befween wide limits — wider in proportion to 

 its sensitiveness, and generally unconformable to the oscillations of the 

 ship. Hence it becomes very difficult to read the average position of 

 the float with a sufficient degree of precision unless the sea be except- 

 ionally calm. * * * 



" With this view the optical densimeter has been devised, which ob- 

 viates all the difficulties arising from the movement of the vessel. The 

 basis of this instrument is the change in the refractive power of a sa- 

 line solution of greater or less density. The instrument consists, sub- 

 stantially, of a hollow prism filled with the water under observation, 

 transmitting from a collimating telescope aline of monochromatic light 

 to an observing telescope, in which the refracted position of that line is 

 read by means of a micrometer. The monochromatic light employed is 

 a sodium flame, obtained by adding a small proportion of a solution of 

 common salt to the alcohol of the lamp. The temperature of the liquid 

 under observation is found by means of a thermometer inserted through 

 the neck of the hollow prism, but which is withdrawn when the optical 

 observation is made. 



"The glass prism rests on three little knobs, so as to have a firm support. 

 Attached to the stand carrying the telescopes are two guides, by means 

 of which the prism is made always to occupy exactly the same posi- 

 tion, so that all observations are made under the same angle. A small 

 thumb-screw on the side of the prism forces it closely into the guides. 

 It is obvious that the sensibility of this apparatus is not affected by 

 the movements of the vessel, and that its power of measurement 

 might be increased, by either enlarging or increasing the power of the 

 telescopes or by introducing an additional prism. But it will be seen 

 at once that the practical accuracy is limited to the ascertainment of 

 the temperature at which the observation is made." 



