Hydrometer

A simple kind of hydrometer is very much used at sea, as “a sea-water test;” and as the observations are usually recorded in a meteorological register or the ship’s log-book, it may not be altogether out of place to give a description of it here.

It is cons

ructed of glass. If made of brass, the corrosive action of salt-water soon renders the instrument erroneous in its indications. The shapes usually given to the instruments are shown in figs. 96 and 97. A globular bulb is blown, and partly filled with mercury or small shot, to make the instrument float steadily in a vertical position. From the neck of the bulb the glass is expanded into an oval or a cylindrical shape, to give the instrument sufficient volume for flotation; finally, it is tapered off to a narrow upright stem which encloses an ivory scale, and is closed at the top. The divisions on the scale read downward, so as to measure the length of the stem which stands above the surface of any liquid in which the hydrometer is floated. The denser the fluid, the higher will the instrument rise; the rarer, the lower it will sink.

The indications depend upon the hydrostatic principle, that floating bodies displace a quantity of the fluid which sustains them equal to their own weight. According, therefore, as the specific gravities of fluids differ from each other, so will vary the quantities of the fluids displaced by the same body when floated successively in each.

The specific gravity of distilled water, at the temperature of 62° F, being taken as unity, the depth to which the instrument sinks when gently immersed in such water is the zero of the scale. The graduations extend from 0 to 40; the latter being the mark which will be level with the surface when the instrument is placed in water, the specific gravity of which is 1·040. In recording observations, the last two figures only—being the figures on the scale—are written down. Sea-water usually ranges from 1·020 to 1·036.

A small tin, copper, or glass cylinder is useful for containing the water to be tested. It should be wider than the hydrometer, and always filled to the brim. If fitted to a stand, which is supported by gimbals, it will be very convenient. Water in a bucket, basin, or other wide vessel, acquires motion at sea, and the eye cannot be brought low enough (on account of the edges) to read off the scale accurately.

Errors of observation may occur with the hydrometer, if it be put into water without being clean, or without being carefully wiped. The instrument is extremely accurate if correctly used. It should be kept free from contact with the sides of the vessel; and all dust, smears, or greasiness, should be scrupulously avoided, by carefully wiping it with a clean cloth before and after use.

Whenever the temperature of the water tested differs from 62°, a correction to the reading is necessary, for the expansion or contraction of the glass, as well as the water itself, in order to reduce all observations to one generally adopted standard.

Negretti and Zambra’s hydrometer, with thermometer in the stem, shows the density and temperature in one instrument.

For the following Tables we are indebted to the kindness of Admiral FitzRoy:—

Table for reducing observations made with a Brass Hydrometer, assuming the linear expansion of brass to be 0·000009555 for 1° F. The correction is additive for all temperatures above 62°, and subtractive for temperatures below 62°.

t°	Correction.	t°	Correction.	t°	Correction.	t°	Correction.
32	-0·0014	48	-0·0010	64	+0·0002	80	+0·0020
33	·0014	49	·0009	65	·0003	81	·0021
34	·0014	50	·0009	66	·0004	82	·0023
35	·0014	51	-0·0008	67	·0005	83	·0024
36	·0014	52	·0008	68	+0·0006	84	·0026
37	·0014	53	·0007	69	·0007	85	+0·0027
38	-0·0014	54	·0006	70	·0008	86	·0029
39	·0013	55	·0006	71	·0009	87	·0030
40	·0013	56	-0·0005	72	·0010	88	·0032
41	·0013	57	·0004	73	·0011	89	·0033
42	·0013	58	·0003	74	+0·0013	90	+0·0035
43	·0012	59	·0003	75	·0014	91	·0036
44	-0·0012	60	·0002	76	·0015	92	·0038
45	·0011	61	-0·0001	77	·0016	93	·0040
46	·0011	62	0·0000	78	·0018	94	·0041
47	-0·0010	63	+0·0001	79	+0·0019	95	+0·0043

Table for reducing observations made with a Glass Hydrometer, assuming the linear expansion of glass to be 0·00000463 for 1° F. The correction is additive for temperatures above 62°, and subtractive for temperatures below 62°.

t°	Correction.	t°	Correction.	t°	Correction.	t°	Correction.
32	-0·0019	48	-0·0012	64	+0·0002	80	+0·0023
33	·0019	49	·0011	65	·0003	81	·0024
34	·0018	50	·0011	66	·0004	82	·0026
35	·0018	51	-0·0010	67	·0005	83	·0027
36	·0018	52	·0009	68	+0·0007	84	·0029
37	·0017	53	·0008	69	·0008	85	+0·0031
38	-0·0017	54	·0008	70	·0009	86	·0032
39	·0017	55	·0007	71	·0010	87	·0034
40	·0016	56	-0·0006	72	·0012	88	·0036
41	·0016	57	·0005	73	·0013	89	·0037
42	·0015	58	·0004	74	+0·0014	90	+0·0039
43	·0015	59	·0003	75	·0016	91	·0041
44	-0·0014	60	·0002	76	·0017	92	·0042
45	·0014	61	-0·0001	77	·0018	93	·0044
46	·0013	62	0·0000	78	·0020	94	·0046
47	-0·0013	63	+0·0001	79	+0·0021	95	+0·0048

158. NEWMAN’S SELF-REGISTERING TIDE-GAUGE.

At places where the phenomena of the tides are of much maritime importance, a continuous series of observations upon the rise and fall, and times of change, is essentially necessary as a basis for the construction of good tide tables; and as such observations should also be accompanied with the registration of atmospheric phenomena, we have no hesitation in inserting a description of an accurate self-registering tide-gauge.

The tide-gauge, as shown in the illustration, consists of a cylinder, A, which is made to revolve on its axis once in twenty-four hours by the action of the clock, B. A chain, to which is attached the float, D, passes over the wheel, C, and on the axis of this wheel, C (in about the middle of it) is a small toothed wheel, placed so as to be in contact with a larger toothed wheel carrying a cylinder, E, over which passes another smaller chain. This chain, passing along the upper surface of the cylinder, A, and round a second cylinder, F, at its further end, is acted on by a spring so as to be kept in a constant state of tension. In the middle of this chain a small tube is fixed for carrying a pencil, which, being gently pressed down by means of a small weight on the top of it, performs the duty of marking on paper placed round the cylinder the progress of the rise or fall of the tide as the cylinder revolves, and as it is drawn by the chain forward or backward by the rise or fall of the float. The paper is prepared with lines equidistant from each other, to correspond with the hours of the clock, crossed by others showing the number of feet of rise and fall.

The cylinder while in action revolves from left to right to a spectator facing the clock, and the pencil is carried horizontally along the top of the cylinder; and the large wheel being made to revolve by the rise and fall of the float, turns the wheel with the small cylinder, E, attached to it. If the tide is falling, the small chain is wound round the cylinder, E, and the pencil is drawn towards the large wheel; but if the tide is rising, the small chain is wound on to the cylinder, F, by means of the spring contained in it, which constantly keeps it in a state of tension. Thus, by means of the rise and fall of the tide, a lateral progress is given to the pencil, while the cylinder is made to revolve on its axis by the clock, so that a line is traced on the paper showing the exact state of the tide continuously, without further attention than is necessary to change the paper once every day, and to keep the pencil carefully pointed; or a metallic pencil may be used, which will require little, if any, attention.

A good self-registering tide-gauge is a valuable and important acquisition wherever tidal observations are required, and the only perfectly efficient instrument of this kind is that invented by the late Mr. John Newman, of Regent Street, London. It is now in action in several parts of the world, silently and faithfully performing its duty, requiring no other kind of attention than that of a few minutes daily, and thus admitting the employment of the person on any other service whose duty it would otherwise have been to have registered the tide. It has done much by its faithful records in contributing to the construction of good tide tables for many places; for those unavoidable defects dependent on merely watching the surface on a divided scale are set aside by it, all erroneous conclusions excluded, and a true delineation of Nature’s own making is preserved by it for the theorist.