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Among the phenomena attending the more tranquil conditions of the air, I had noticed in my earlier observations, during the summer of 1857, that upward currents generally prevailed by day, while downward currents became more prominent at night. This alternation was manifestly connected, as shown by the horizontal vane, with the action of land and sea breezes ; for at this time the observations were made at a point situated about two miles from the sea-shore. By day, the convection due to the heating of the lower stratum of air in contact with the ground could not take place by equal upward and downward exchanges of masses of air, because the place of the ascending warm air was partly supplied by the lateral influx of colder sea air, which, in its turn, would become sufficiently heated to ascend and give place to a fresh lateral influx. By night, the colder air from the land flowed towards the sea, and its place was filled by descending currents from above. At the same time the warmer air from the sea probably tended to occupy the place of these currents, and thus to equalize the temperature of the upper and lower strata of air so as to lessen the energy of the convective movement over the land.
Before the termination of the Meeting of the Association at Manchester, I had resolved, with the concurrence of Mr. Glaisher, the only other member of the Committee then present, to cause a registering instrument to be constructed which would record the existence of non-horizontal atmospheric motions. The following is a description of the anemoscope which I ultimately decided upon as most suitable in its construction for the purposes we have in view. Fig. 1 is a vertical section of the portion of the apparatus which is exposed to the wind, and fig. 3 an elevation of the same portion. A is a cast-iron pillar which supports a cup, h, containing friction-balls made of gunmetal; on these a disk, g, rests, and this is firmly attached to a box from which an arm projects at one side, and is terminated by the cone, P, which acts as a counterpoise for the opposite and working arm of the anemoscope. A short arm, n, shown in fig. 3, supports a wheel, d, in one side of which teeth are cut; the other side is firmly attached to a hollow light copper box, B, which forms the tail. This box is a truncated pyramid, and while its vertical sides are exposed to the horizontal action of the wind, its upper and lower surfaces are exposed to its vertical action. This tail is balanced by a counterpoise, t, which is connected by a bent arm with tho axle of the wheel, d. The teeth of this wheel catch those of the pinion, e (fig. 1), and this catches in the rack, /. The rack is attached to a shaft, c, which descends through the hollow supporting pillar and communicates with the registering apparatus. In fig. 2 tho most essential part of the arrangements for registering the indications of the upper part of the instrument are shown. The shaft, e, passes through brass guides, and carries a small circular projecting piece, s, which catches in a notch made in the bit, v, attached to the pencil-carrier,^. This pencil-carrier is capable of upward and downward motions only, and the rod to which it is attached passes through guides. The carrier is, moreover, supported by an ivory friction-wheel, t, which turns when the piece, s, revolves beneath it.
From this brief description, it is apparent that the cone, P, will always indicate the direction of the wind in azimuth, like ordinary vanes. At the same time the vertical component (if any) of the wind will raise or depress the tail, B. In the former case it is manifest that the wheel, d, will cause e to turn, so as to raise the rack,/, and in the latter case the effect will be to lower the rack. It follows, therefore, that the shaft, c, and consequently the pencilcarrier which it moves, must rise or fall according as the vertical motion of the air is upward or downward. A spring within the pencil-carrier constantly presses the pencil against a sheet of paper placed in front of it. This paper is for the present carried on a flat board, which is moved by a clock. The registering sheets are ruled with vertical hour lines and with horizontal
lines which assist in estimating the angle of inclination to the horizon made by the disk during the action of an upward or downward impulse from the air. This follows because the tail and the wheel, d, revolve on the same centre, and each tooth in d describes an arc similar to that described by the axis of the tail. An equal number of teeth in e are raised or lowered, and thus the rack and the shaft, c, move through spaces proportional to arcs described by the teeth of the wheel, d, and the axis of the tail, B. The board which carries the registering paper can be detached by loosening a clampingscrew which fastens it to the support turned by the clock, so that the sheets can be removed and replaced with speed and facility.
The entire' apparatus was constructed by Mr. Spencer, of Aungier Street, Dublin; and he has executed the portion connected with the indication of horizontal movement in such a way, that the addition of a registering apparatus for this part of the instrument will not only be easy, but will render the entire combination a complete indicator of the absolute direction of the wind. The results of the instrument in its present state are exhibited on the registering sheets as nearly vertical pencil lines, some above and some below the neutral line, to which each sheet is carefully adjusted.
The anemoscope is at present so placed as not to be overtopped by any building; for it stands on the roof of one of the highest houses in Dublin, in a quarter remarkably open, and close to the south suburbs.
Owing to a variety of delays and obstacles in finishing the apparatus, it was not brought into action until the 31st of August, and thus I am ablo to report only on the results furnished by little more than the records of a single month. These records appear to indicate that vertical oscillations prevail more during the mid-day hours than at other periods; for although ten sheets show no definite predominance at any specific period of the day, and two predominance of vertical movements towards midnight, twenty-one show that these movements are most frequent at the hours about noon. From a journal of the weather which was kept at the. same time, it appeared that on bright days, when the air had little horizontal motion, gentle upward movements prevailed at mid-day. Such phenomena are distinctly manifested by the sheets for September the 5th, 6th, 7th, 8th, and 9th, and all of these were bright sunny days. Before the 5th, the weather had been changeable and unsettled: but on comparing the two sheets comprehending from noon of the 3rd to noon of the 5th, I noticed that the amplitude of the oscillations of the anemoscope progressively and regularly diminished; and it occurred to me that this might indicate a tendency towards convective equilibrium of the atmosphere, and more settled weather. The weather continued fine until the 13th, when there was both high wind and rain, accompanied and preceded by energetic oscillations of the anemoscope. If the general circulation of the atmosphere takes place, as seems to bo now completely established, by a twofold motion, one of translation, whether cyclonic or lineal, and the other undulatory, it follows that the pulsations of the latter movement may be influenced by aerial disturbances. The frequency, regularity, intensity, prevalent direction, and more or less intermittent character of these pulsations must depend on variations of pressure, density, moisture, and temperature, as well as on the rippling motion of the air. It is natural, therefore, to expect, what our limited number of observations seem already to indicate, namely, that the sudden and abrupt commencement of such pulsations is usually a precursor of other disturbances, while their gradual and regular diminution in energy would show a tendency in the air to approach a stato of convective equilibrium, and might, therefore, be safely relied upon as a forerunner of fine weather. This point is illustrated by the remarks of the late Professor Daniell relative to the rapid oscillations of the water-barometer during high winds, and their gradual diminution preceding a return to a calmer state of the air*. Although the atmospheric pulse is undoubtedly compounded of the undulatory movements resulting from the flow of an elastic fluid over the
* Phil. Trans. 1832, p. 673.
irregularities of the earth's surface, with the effects of convection, in such a way as would render the separation of these effects extremely difficult, yet the careful study of this pulse in connexion with other phenomena may he reasonably expected to add to our power of forming correct conclusions regarding the coming changes of the weather.
Report of a Committee, consisting of the Rev. Dr. Lloyd, General SaBine, Mr. A. Smith, Mr. G. Johnstone Stoney, Mr. G. B. Airy, Professor Donkin, Professor Wm. Thomson, Mr. Cayley, and the Rev. Professor Price, appointed to inquire into the adequacy of existing data for carrying into effect the suggestion of Gauss, to apply his General Theory of Terrestrial Magnetism to the Magnetic Variations.
In order to explain the views of the Committee upon the question submitted to them, it is necessary to refer briefly to the leading points of Gauss's theory.
If dp denote the quantity of free magnetism in any element of the earth's mass, and p the distance of that element from the point (x, y, z), and if we make
the partial differential coefficients of V with respect to the three coordinates, x, y, z, respectively, are equal to the components of the earth's magnetic force in the direction of the axes of coordinates. V is a function of x, y, and z, or of their equivalents u, X, and r,—r being the distance of the point from the centre of the earth, and u and X the angles corresponding to the north polar distance, and the longitude, on the sphere whose radius = r. This quantity may be expanded in a series proceeding according to the inverse powers of r, whose coefficients, P,, P2, P3, &c, are functions of u and X alone; and it is readily seen that, at the surface of the earth, the three components of the magnetic force are
\au clu clu J
sin u \ d\ d\ a\ J
Z=2P1 + 3P2 + 4P3 + &c,
and are therefore given when P,, P2, P3, &c. are known.
The form of these functions is deduced from the well-known partial differential equation
n (n+1) Ps+£i!-+cott* + -J— ~P=0,
n being the number indicating the order of the function. It is found that the first, P,, contains three unknown coefficients; the second, PJ( five; the third, P3, seven, &c. Hence, if the approximation be extended so as to include terms of the fourth order, there will bo 24 coefficients to be determined. Each given value of X, Y, or Z, on the earth's surface, furnishes an equation among these unknown coefficients; and for each place at which the three elements are known we have three such equations. Hence to obtain the general expressions of X, Y, Z, to the fourth order inclusive, it is theoretically sufficient to know the three elements at eight points on the earth's surface. But, owing to the errors of observation, and to the influence of the terms neglected in the approximation, the number of determinations must, in practice, be much greater than the number of unknown coefficients.
The foregoing conclusions are based upon the hypotheses that magnetic attraction and repulsion vary according to the inverse square of the distance, and that the magnetic action of the globe is the resultant of the actions of all its parte. It is likewise assumed that there are two magnetic fluids in every magnetizable element, and that magnetization consists in their separation. But for these hypotheses we may substitute that of Ampere, which supposes the magnetic force to be due to electric currents circulating round the molecules of bodies.
This theory may be applied to the changes of terrestrial magnetism, whether regular or irregular, provided only that the causes of these changes act in the same manner as galvanic currents, or as separated magnetic fluids. We have only to consider whether the data which we possess are sufficient for such an application.
It has been already stated that, for the general determination of X, Y, and Z, wo must know their values at eight points (at least) on the earth's surface, these points being as widely distributed as possible. The same thing holds with respect to the changes £X, 3Y, 2Z; and to apply the formula? so determined, and to compare them with observation, corresponding values must be known for (at least) one more point. In the case of the irregular changes these observations must, of course, be simultaneous. The regular changes must be inferred from observations extending over considerable periods; and there is reason to believe that these periods must be identical, or nearly so, for all the stations, since the changes are known to vary from month to month and from year to year.
The regular variations of the three elements X, Y, Z, or their theoretical equivalents, have been obtained by observation, for nearly the same period, at Greenwich, Dublin, and Makerstoun, in the British Islands; at Brussels and Munich, on the Continent of Europe; at Toronto and Philadelphia, in North America; at Simla, Madras, and Singapore, in India ; and at St. Helena, the Cape of Good Hope, and Hobarton, in the southern hemisphere. Of these thirteen stations, however, the three British must be regarded, for the present purpose, as equivalent to one only, on account of their proximity; and the same thing may be said of the two North American stations and of the two stations in Hindostan. This reduces the number of available stations to nine, the minimum number required for the theoretical solution of the problem in the degree of approximation already referred to, and considered by Gauss to be necessary. It is true that we may add to these the stations at which two only of the three elements have been observed, viz. Prague and St. Petersburg, the three Bussian stations in Siberia, and Bombay. But even with this addition, the number is probably insufficient for the satisfactory determination of the unknown coefficients; for it is to be remembered that the places, few as they are, are not distributed with any approach to uniformity, and that very large portions of the globe are wholly unrepresented by observations.
For the reason already stated, this defect in the existing data cannot be now repaired by supplemental observations at new stations, unices the series