of Edinburgh, Session 1884-85. 
35 
2. Determination of the Density. 
The density of the samples of water collected in the firth is 
determined by means of a very delicate hydrometer of the form 
used on hoard the “Challenger.” 
The hydrometer is made of glass, the tubes for body and stem 
having been very carefully selected to ensure uniformity of dia- 
meter. The instrument has a body of about 5 centimetres 
diameter, and 12 centimetres long; the stem is nearly the same 
length, and has a diameter of 3 millimetres. The process of making 
and calibrating the hydrometer has been described in great detail 
by Mr Buchanan, in his “ Challenger Report on the Specific 
Gravity of Ocean Water ” (Chall. Rep. Phys. Chem ., vol. i. 
pt. ii. pp. 1-4). 
The hydrometer which has been used at the Marine Station is 
provided with seven movable weights, which can be attached to 
the top of the instrument, and thus adapt it for observing densities 
of various amount. The first four weights slip over the top of the 
stem, and rest there, the others are ring-shaped, and are used to 
supplement the weight of the former. The hydrometer' weighs .in 
vacuo 150*1478 grammes. The weights have the following values : — 
1. 4*0285 2. 3*2247 3. 2*4099 4. 0*8195 
5. 0*8125 6. 0*5184 7. 0*3312 
As each of the first four weights can be used with any or all of 
the remaining three, and as, with a little trouble, any of the last 
three can be balanced on the top of the stem by itself, the total 
weight of the hydrometer may be varied from 150*1478 to 
155*8384 through 36 gradations. 
The volume of the body and bulb of the instrument is 150*2070 
cubic centimetres at 0° C. At 25° C. it is 150*3210, the volume of 
the 100 millimetres into which the stem is divided is 0*85 c.c. ; 
and as it is assumed to be uniform, the volume of each millimetre 
of the stem is taken as 0*0085. 
Most of the density observations were made on the “Ark,” the 
floating laboratory of the Station. As she is perfectly steady, 
except when there is a conjunction of full tide and high wind, it 
was not necessary to use a swinging table. The process of making 
an observation of the density is as follows : — 
