electrician I suppose could be found, who would question the correctness of this; all will admit that the time is dependent on the ability of the pile to generate the electricity, and the extent of the inductric surface (the size of the jar). But is not this phenomena precisely equivalent to the charging of the cable? is the DeLuc's column a poorer generator in proportion to the surface of the jar than an ordinary battery is in proportion to the four acres of cable surface? Every electrician who would have the problem of diminishing the time of charging the jar by the column, would have to resort to methods of increasing the activity of the pile, either by changing the nature of its elements, or increasing their surface; then why does not the same doctrine hold good in regard to charging that big jar, the Atlantic cable? I have endeavored to demonstrate that submerged telegraph lines differ from the land lines in requiring batteries having the plates very large, because the electrical condition of the cable at the first moments of contact are such that require a battery capable of furnishing a current of great quantity as well as great intensity. I have, however, made no indication of the precise size or number of plates which should be used for working the Atlantic cable, or any other submerged conductor, with greatest speed. The limit to the rapidity of working must be determined by the conditions for carrying off the heat generated in the conduction by the passage of the electric current; and as the quantity of electricity which may be used on any cable without endangering the gutta percha insulation by the heat of the conductor can be ascertained only by experiment, it is by this means only, that the best size and number of plates can be determined for working with the greatest speed the cable will admit of.

In the passage of very large quantities of electricity into the cable the heating effect on the portion nearer to the battery will be considerable, for although the measure of the heating power of the current is the conduction resistance, and the resistance of the current is destroyed by the induction "masking" the electricity; yet as the portions of the cable nearer to the battery are first charged, and the conductor, after the dielectric is charged, resists as an ordinary conductor, it is evident that those portions will be heated by the current; yet the whole quantity of heat produced will be less than if no inductive action took place. Considering the whole quantity of heat generated by the oxydation of a given quantity of zinc, or the resistance to the electricity thereby generated, which is the same thing, and the greatness of the surface to carry off that heat, it is easy to foresee that a very large quantity* of electricity might be used on the

* While a quantity of electricity even insufficient to charge the whole cable might rupture it if in a state of very great tension, yet no danger of this need be apprehended from any quantity of electricity obtained from any easily attainable number of voltaic elements.

cable without endangering it, and consequently a high speed in transmission obtained. For a first trial on the Atlantic cable I would use a Smee battery of about 500 pairs of plates, each having 20 square feet of surface; yet it may be found on trial that plates of much larger size may be required.

Although the present cable may be incapable of working with a satisfactory degree of certainty and rapidity, even with the best electrical management, yet this would not demonstrate that there cannot be a satisfactory electrical communication across the Atlantic. Certainly I will be justified in saying that the present state of the engineering art and the experience already obtained in laying submarine conductors, can lay down a new cable capable of transmitting 100 letters per minute, if it is constructed and worked in full regard to the principles of electricity.

ART. XVIII.-On the Variation of the Magnetic Needle at Hudson, Ohio; by ELIAS LOOMIS, Professor of Mathematics and Natu ral Philosophy in the University of the City of New York.

summer.

DURING my residence at Hudson, Ohio, between the years 1837 and 1844, I made repeated observations for the purpose of determining the variation of the magnetic needle. During the summer of 1849, while employed in determining the longitude of Hudson by telegraphic comparisons with Philadelphia, I repeated my observations for the variation; and Prof. C. A. Young, of Western Reserve College, has put into my hands a very complete series of observations made by himself during the past All these observations were made with a variation compass by Gambey of Paris, having a needle about 18 inches long, supported by fibres of untwisted silk, and resting in a stirrup which admits of easy reversal. It is now proposed to compare these observations for the purpose of determining the annual change of the variation. As these observations were made at different hours of the day, it is necessary to apply a correction to reduce each result to the mean variation of the month of observation. As the observations required for this purpose have never been made at Hudson, or at any place in its immediate vicinity, we must rely upon observations made at a considerable distance. The nearest station at which such observations have been made is Toronto, which is north of the parallel of Hudson. On the southern side we have observations both at Philadelphia and Washington. I have preferred the latter, since Washington is nearer to Hudson than Philadelphia; and the parallel of Hudson is exactly midway between the latitudes of Toronto and Washington.

The following table shows for six months of the year, the quantity by which the variation at the hour named in column first, differs from the mean variation for the month at Toronto. The table is derived from five years observations between 1843 and 1848, and is copied from page 90, vol. iii, of the Toronto observations.

Diurnal change of the Magnetic Variation at Toronto.

The following table shows for the same months the quantity by which the variation at the hour named in column first, differs from the mean variation for the month at Washington. It is derived from two years observations from 1840 to 1842, and is deduced from the table on page 326 of Gilliss' Magnetical Observations.

Diurnal change of the Magnetic Variation at Washington.

The following table shows the observations of the magnetic variation at Hudson. Column first shows the day of the month and year; column second the hour of observation; column third the observed variation; column fourth the correction applied to reduce the observation to the mean for the month; and column fifth shows the variation thus corrected. The correction is obtained by taking the mean of the numbers furnished by the Washington and Toronto observations.

In order to deduce from these observations the most probable result, let us put & for the mean variation Jan. 1, 1839, and ▲ for the annual motion. Then if we regard the observations of any one year as forming a single observation, we shall have the following eight equations of condition.

d

=

Solving these equations by the method of least squares, we obtain 55'213, which is the mean value of the variation for Jan. 1, 1839; and A=-22416; that is, the easterly variation is diminishing at the rate of 24 minutes in a year. We also conclude that the line of no variation will pass through Hudson sometime during the year 1863.

ART. XIX.-On the Dynamics of Ocean Currents; by Lieut. E. B. HUNT, Corps of Engineers, U. S. Á.

(Read before the American Association, at the Baltimore Meeting, May, 1857.)

It can scarcely be denied that the state of our knowledge of ocean currents is any thing but satisfactory. Not only are we to a very great extent ignorant of the precise state of the facts, but we are also deficient in the theoretical exposition of those already known. We can easily explain our lack of precise knowledge of facts by reference to the circumstances. The vast oceanic areas can be observed only by persons engaged in navigation, who are mostly unfurnished with proper means for correct determinations, and who lack that special training which is a prime

essential for good observations. The facts to be observed are also of a character so complex and elusive, are so subject to fluctuation in a given locality, and are involved in movements of air and water of so vast compass, that we cannot hope for precision of knowledge by the use of means now in operation. The single fact that most observations are made on the water surface while the ocean depths are of vital efficacy in shaping all marine phenomena, gives a character of signal incompleteness to those observations which have been mainly instrumental in fixing the received notions on the system of oceanic circulation. It is a fit subject of regret that the discussion of ocean movements has been so rarely attempted by those whose previous training in mathematical or mechanical science would have been a sufficient guaranty and preventive against the wild and illogical rhapsodies of theorists who have run riot over the broad domain of the physics of the sea.

With a view to apprehending the mechanical elements of this problem of ocean currents, let us first suppose a terrestrial sphere, which has assumed the equilibrium condition, resulting from gravitation, diurnal rotation, a solid nucleus and a homogeneous water envelope unbroken by land. This water stratum would shape itself so that its bounding surface would be a strictly mathematical level surface. A level surface of this nature may be defined as one which is at each point perpendicular to the resultant of all the forces acting on the individual molecules situated in that surface. In this case it would be a continuous oblate spheroid to which the resultant of gravity and centrifugal force would be everywhere normal. If to this we add those diurnal disturbances of the normal level due to the irregularities of solar and lunar attraction during the earth's rotation, we obtain the tidal waves which appear as perturbations of the normal level. If the continental masses be supposed to be elevated, we have a slightly modified normal level surface for the ocean; but one, which once determined becomes the proper standard of reference for all oceanic perturbations, to whatever cause due. This surface is everywhere the true bounding surface, and cuts the resultant of gravity and centrifugal action for the earth as it is, perpendicularly at each point of the surface, and is entirely continuous, though no more truly spheroidal. This is the normal ocean level, and it is a useful surface of reference for all vertical ocean movements or perturbations. If we now suppose the homogeneous earth without continents subjected to the heating action of the sun's rays, the result will be that the equator will become a line of maximum heating, from which to the poles there will be a progressive diminution of heat absorption. This would cause an expansion of the heated waters which would thus rise above the normal level surface by an amount equal to