[27] 



CONSTRUCTION AND WORK OF FISH-HAWK. 



29 



readiug of a unit in tlie fourth pliice of decimals by estimation. The 

 float is accompanied by a copper can, with a thermometer inserted 

 within the cavity, which is glazed in front. In use the can is nearly 

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

 ing being then taken with ease at the top of the liquid. For conven. 

 ience 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 sawdust, is kept on hand to replace the broken ones. 



The following table has been derived from the observations of the 

 ex[>ansibility of sea- water, made by Prof. J. S. Hubbard, U. S. N. Col- 

 umn II contains a table of reductions for temperature of salinomet<?r 

 readings to the standard of 60° Fahr. To facilitate the use of this table 

 the following directions are given: 



Record the actual observation of hydrometer and thermometer. 

 From Column II (which is applicable to any degree of saltness within 

 the given limits) take the number corresponding to the observed tem- 

 perature, and multiply this number by the number of degrees and frac- 

 tions of a degree that the observed temperature differs from 60°. Ap- 

 ply this i)roduct as a correction, with proper sign, to the reading of the 

 vsalinometer, and the result will be the readiug of the salinometer at 

 the standard temperature of 60° Fahr. 



Example. — Actual reading of thermometer =80^.5 ; actual reading of 

 salinometer =1.02425. 



Oi)posite 80O.5 in column II is +0.0001585, which, multiplied by 20.5, 

 gives as a product -f-0.00o249. Add this to the observed reading ot 

 salinometer, and 1.02750 will result as the reading of the salinometer at 

 the standard temperature. 



A method quite diflerent in practice for determining the density of 

 sea- water has been suggested by Prof. Wolcott Gibbs, of Harvard Uni- 

 versity. It depends upon the determination of the index of refraction 

 by means of an angular instrument similar to the sextant. As all navi- 

 gators are familiar with thp use of the sextant, and as the observation 

 can be made without hinderance from the motion of the ship, this form 

 of the instrument may be found to possess certain advantages. 



