list of altitudes of 150 carefully located points. Those especially interested in the paper will find it in the Report of the United States Geological and Geographical Survey under Professor F. V. Hayden for 1873.

THE BATHOMETER OF DR. C. W. SIEMENS, F.R.S.

In a report to the Secretary of the Navy on the Loan Exhibition of Scientific Instruments at South Kensington, by Professor E. S. Holden, U. S. N., are some interesting remarks upon this ingenious instrument.

Professor Holden says: The best description of this instrument is to be found in Nature for March 30, 1876, from which the following account is taken: A paper has been presented to the Royal Society "On determining the depth of the sea without the use of the sounding-line," by Dr. C. W. Siemens, who gave at the meeting of the 24th of February, 1876, a description of the instrument which he has designed with this object. He commenced by giving a mathematical statement of the effect of local attraction, to a certain depth, on a body placed at the surface of the earth, assuming it to be of uniform density, spherical in form, and unaffected by centrifugal action. For small values of depth (h) this attraction is 2h. The original formula from which this is adduced is:

and by substitution of 2R for h in this, Newton's statement

4

of the total attraction is obtained.

Now, if in place of the solid substance which forms the exterior crust of the earth, whose density may be taken to be the mean density of superficial rock, water, a material of less density, is substituted, it is shown that the total attrac tion must be diminished, and the measure of this diminution is a measure of the depth of light substance which has been substituted for heavy. If we were in possession of the exact mean density of the earth, of that of the surface-rock, and of sea-water, a scale could be calculated beforehand to show what depth would agree with a certain diminution of the measured effect of gravitation. Such an approximate

calculation was made in designing the instrument, but Mr. Siemens has preferred to compare the readings of the instrument with actual soundings, in order to obtain a scale.

The instrument, which is called a bathometer, consists of the following parts: A weight, being a column of mercury affected by variation of gravitation; a counterbalance, being springs unaffected by variation of gravitation; and an ar rangement by which variations in gravitation can be read as depths in units. The column of mercury is maintained in a vertical steel tube having cup-like extensions, the lower por tion being closed by a corrugated diaphragm of thin steel plate, and the upper portion containing an aperture for filling the instrument, having a screw stopper. The internal diameter of the tube is reduced at the upper portion, in order that the vertical oscillations of the mercury produced by the motion of a vessel in a sea-way may be reduced to a minimum; and the instrument is suspended in a universal joint above its centre of gravity, so that it may always hang in a vertical position at sea, and is inclosed in an air-tight casing, so that it may not be under the influence of atmospheric changes. The weight of the column of mercury is balanced at the centre of the diaphragm by the elasticity of the steel springs, and the modus operandi of the instrument is evident; as the mercury diminishes in potential through the effects of diminished attraction, the action on the springs diminishes, and these shorten upon themselves.

There are some peculiarities in the mechanical arrangement of the instrument which repay examination. Both ends being open to the air, its indications are not affected by variations of atmospheric pressure. With regard to temperature, the instrument is parathermal.

Professor Holden states that by the courtesy of Dr. Siemens he was enabled to see the manuscript account of soundings taken with this instrument on board the Faraday in October and November, 1875, and in March and April, 1876. During her voyage across the Atlantic at that time frequent soundings were made with the piano-wire sounding apparatus of Sir William Thompson, and at each one of these soundings the bathometer was read by Dr. Hicks, who had it under his charge. The results are exhibited in the following table:

The following soundings were taken in March and April, 1876, by Alexander Siemens, Esq., near Nova Scotia. Some sources of error had in the meanwhile been removed:

It will be seen from the table that the mean error is a lit tle over eight per cent. for the first trials, during which time the instrument (which was the first of its kind) was subject to various sources of error, some of which have now been removed by better mechanical arrangements, and a few of which remain to be overcome. It will also be observed from the second part of the table that in April, 1876, after certain of these mechanical difficulties had been overcome, the percentage of error was much reduced, so that a sounding by the instrument and one by the line agreed on the average to within about four per cent. Dr. Siemens is now engaged in incorporating some improvements into a new instrument of this kind, and in particular in substituting for the corrugated diaphragm, which supports the column of mercury, one made of steel which is sawed through in a spiral form and covered by a sheet of India rubber.

It

In the trial of this first bathometer, Dr. Siemens took it to the top of the great clock-tower of the Houses of Parliament (315 feet), and it was found to read very closely what theory demanded. It may be reasonably hoped that the mechanical difficulties, which are very great, can be so far overcome as to cause the instrument to be read in the open sea within one or two per cent. of the true depth on the must be remembered that the piano-wire apparatus gives the depth of water immediately below the ship, while the ba thometer gives an indication of an attraction (or of a defici ency) which is the result of the depth of water for several miles in every direction. It gives the mean depth over a certain area.

average.

A difficulty will always arise in the use of this device as a navigating instrument in general, as on approaching close to the land it will give an indication due not only to the depth of water beneath the ship, but partially due to the height of the neighboring land above the level of the sea. Thus we may expect to find always a reading of the instru ment near the shore which will be different from what it would give in the open sea over the same depth of water, and hence, for general cruising, such an instrument will be an uncertain guide of a close approach to land. When, however, the water shoals gradually, so that the 100-fathom line is twenty-five to forty miles from shore in which case the

disturbing effect of the distant shore would be very small, it would indicate with great certainty the shoaling of the. water, and a simple observation of the rate of shoaling could not fail to be of service to the navigator.

The foregoing objections would not apply, however, in a case where the instrument was placed on a packet-ship which was continually making the same voyage, as on one of the Atlantic steamers for example.

If whenever the ship's position was known the bathometer was read and the reading entered on the chart, and if after ten or twenty voyages a particular instrument was constantly seen to have the same reading when the ship was in a given position (as it undoubtedly would), it could not fail to be trusted as a warning and as an aid. By an experience of this kind a table of the bathometer readings which corresponded to particular positions could be made and thoroughly tested on such a vessel, which is constantly crossing in the same track, so that after a time the reading of this particular bathometer would become an important sign of nearing

a coast.

In this way, with intelligent officers who would not fail to put the necessary tests to it, it may yet be deemed suitable to serve as an important aid to navigation. Its great use will be, for the present, on board of special surveying ships, to indicate not so much the actual and absolute depths as the changes of depth. If, for example, a ship is engaged in making soundings with a trustworthy apparatus on board, and if it is found that a given depth by the piano-wire always corresponds very nearly to the same reading of the instrument (as it would), and if this continues to be so, then a change in the reading of the bathometer would indicate to the commanding officer the passage over shoaler or deeper water, as the case might be, and it would show the necessity for a new sounding-a necessity which otherwise would be overlooked. In the hands of intelligent officers this instrument, which is now an experiment merely, may become of great value, and it is quite within reasonable expectations to hope for valuable aid from it in its perfected form.

It is understood that an instrument of this class will soon be in the possession of the Navy Department, and one has been proposed for use on H. M. S. Fawn, and upon the re