NARRATIVE OF THE CRUISE. 



109 





of the liquid. This was effected satisfactorily by the use of a hydrometer (fig. 37), 

 specially designed by Mr. Buchanan for the purpose. 1 



The following is a description of the instrument used for the whole of the work 

 done during the cruise. The stem, which carries a millimetre scale 10 

 centimetres long, has an outside diameter of about 3 millimetres, the 

 external volume of the divided portion being 0"8650 cubic centimetre ; 

 the mean volume of the body is 160 - 277 c.c, and the weight of 

 the glass instrument is 1 60*2128 grammes. With this volume 

 and weight, it floats in distilled water of 16° C. at about the lowest 

 division (100) of the scale. In order to make it serviceable for denser 

 waters, a small brass table is made to rest on the top of the stem, of 

 such a weight that it depresses the instrument in distilled water of 16° C. 

 to about the topmost division (0) of the scale. By means of a series 

 of six weights, multiples by 1, 2, 3, 4, 5, and 6, of the weight of the 

 table, specific gravities between 1 "00000 and T03400 can be observed. 

 It is not necessary that these weights should be accurate multiples of the 

 weight of the table ; it is sufficient if they approach it within a few 

 milligrammes, and their actual weight be known with accuracy. The 

 weights of the table and of the weights in actual use were : — 



Weight of table, . 



,, of weight No. I. 

 II. 



HI. 

 IV. 



v. 



VI. 



0-8360 grammes. 



0-8560 



1-6010 



2-4225 



3-2145 



4-0710 



4-8245 



For oceanic waters, the hydrometer is always used with the table, 

 and either No. IV. or No. V. weight. 



For using this instrument at sea, about 900 c.c. of sea water are 

 taken, and the containing cylinder placed on a swinging table, in a 

 position as near the centre of the ship as possible (fig. 38). The 

 observation with the hydrometer, loaded with the necessary table and 

 weight, is then effected in the ordinary way, the accuracy of the readings 

 being but little affected by rolling ; pitching, however, is found to have 

 a distinctly disturbing effect, and when it is in any way violent, it is 

 advisable to store the specimen of water till the weather improves. 



The temperature of the water at the time of observation is determined by one of 



Fig. 37. -Hydro- 

 meter. 



1 Phys. Chem. Chall. Exp., part ii., 1884. 



