THE VOYAGE OF H.M.S. CHALLENGER. 
A hydrometer of the delicacy required for our work has necessarily a very limited 
range, otherwise its dimensions would be inconveniently large. In order to avoid the 
necessity of taking a number of different instruments adapted to different ranges of 
density, a number of brass weights capable of being attached to the top of 
the stem of the hydrometer were provided. By this means the weight of 
the hydrometer could be varied at will within certain limits. One of these 
weights was in the form of a small brass table, and when more were 
required they were laid upon it. 
The instrument which was used during the whole cruise answers 
perfectly the purpose for which it was designed, and may here be particularly 
described (fig. 1). Preliminary calculations showed that convenient dimen- 
sions would be about 3 mm. for the diameter of the stem and about 150 c.c. 
for the volume of the body, and from 10 to 12 cm. for the length of the stem. 
The tube for the stem was selected with great care from a large assortment, 
and no want of uniformity in its outward shape could be detected with the 
callipers. The tube for the body of the instrument was also selected from 
a number, in order to secure such a diameter as would give the instrument 
a suitable length. In order to provide against accidents, I had four instru- 
ments made from the two lengths of tubing. The glass work of the instru- 
ment being finished — except that the top of the stem, instead of being sealed 
up, was slightly widened out into a funnel — the instrument was loaded with 
mercury, until the lower end of the stem was just immersed in distilled 
water of 16° C. A millimetre scale on paper was then fixed in the stem, 
and the calibration carried on by placing decigramme weights on the 
funnel-shaped top, and noting the consequent depression on the scale. 
The whole length of the scale was 10 cm., and this portion of the stem 
proved to be of perfectly uniform calibre. Several series of observations 
were made in order to determine accurately the volume of any length of 
the stem. 
Table I. gives the results of two series of these observations. Pure 
distilled water of the temperature of the laboratory (1G° C.) was used. 
The temperature of the water was carefully checked with one of 
Gicivder's standard thermometers, divided into tenths of a centigrade 
degree ; it varied from 1G°*0 to 16 '2 C. We may therefore take the mean 
temperature to be 16 *1 C. In the first series the mean depression per 
decigramme is 11 ‘525 mm.; in the second it is 1 1 *587 mm. 
of th' -e i- 11*556 mm. This length of stem is immersed by 0‘1 grm.; the volume 
therefore of 1 1 *550 mm. of the stem is equal to that of 0*1 grm. water at 16°*1 C., 
or 0* 100007 c.c. Hence the volume of the graduated portion of the stem (100 mm.) is 
Fio. 1. — Completed 
The mean Hydrometer. 
