tent and in the degree which it ought to do in virtue of the earth's diurnal motion.

SPHEROIDAL FORM OF THE EARTH.

Although the earth be said to be a globe in the ordinary sense of the term, and when extreme accuracy is not sought, yet, strictly speaking, it deviates from the globular form. It has been ascertained that its figure is that which in geometry is called an oblate spheroid. To acquire a notion of this form, we have only to imagine an oval, such as A B C D, fig. 11, to revolve upon its short axis B D. The figure it would produce by such a revolution would be an oblate spheroid. It will differ from that of a sphere, inasmuch as the polar diameter B D will be shorter than the equatorial diameter A C.

A familiar example of this figure is presented by a turnip, or in a less exaggerated form by an orange.

The degree in which the earth has this peculiar form is, however, so very slight, that if we made a model of it in a lathe, the eye could not discover that it was not a true globe. Its oblateness could only be detected by accurate measurement, or by causing it to revolve in different positions in the lathe, and applying to it a tool fixed on a rest. In fact, the equatorial diameter of the earth is to the polar diameter in the proportion of three hundred and one to three hundred; or, in other words, the diameter of the equator exceeds the length of the polar axis by one part in three hundred. If, then, we take in round numbers the polar diameter to be eight thousand miles, we shall find the equatorial diameter to be eight thousand and twenty-six miles; thus the parts of the earth's surface at the equator are twenty-six miles further from the centre of the earth than the parts near the poles.

Such being understood to be the real figure of our globe, it will be asked how it has been ascertained to be so. This question may be examined in either of two ways-either as one of theory or one of fact. We may show, that, from the known laws of mechanics, a globe like the earth revolving on an axis in twenty-four hours, must become an oblate spheroid of the above dimensions; or we may show by measurements made on different parts of the earth's surface, that it is, in fact, such a spheroid, whatever cause may have imparted that figure to it.

It is well known that when any particle of matter revolves in a circle, it has a tendency to recede from the centre of the circle, in virtue of what is called centrifugal force. Now all points on the surface of the earth revolve very rapidly in circles by reason of the diurnal motion of the globe. Any point, for example, on the equator, revolves in a circumference of twenty-five thousand miles in twenty-four hours. A point at a higher latitude revolves in the same time in a less circle; and the circles of diurnal revolution become gradually

less and less as we approach the poles. Since, then, the centrifugal force depends conjointly on the magnitude of the circle of revolution and the velocity of the motion, it follows that it will be less and less as we approach the poles, and greater and greater as we approach the equator.

This force, however, exists at all latitudes, in a greater or less degree of energy, and it is everywhere directed from the centre of the circle of diurnal rotation. Let NO S, figure 12, be the earth, and E Q the equator.

Let P be a point on the surface of the earth anywhere between the equator and poles. Since P is carried by the diurnal motion round the centre C, it will have a tendency to fly from the centre in the direction P R. This tendency will be partially counteracted by its gravity, which acts in the direction PO. But since P O is not directed immediately against P R, the result will be that a particle of matter P thus acted on will move toward Q. To counteract this tendency, there must be such a protuberance at Q as will place an acclivity before P so steep as to prevent its ascent. Without such a protuberance, all the fluid and loose matter on the globe would run toward the line.

It appears, then, that the effect of the earth's revolution would be to cause all loose matter placed on the surface of the earth in either hemisphere to move toward the equator; and that if the earth were a perfect globe, there would be no power to resist this tendency, and the effect would consequently be actually produced.

Let us, then, suppose an exact globe, partially covered with land and water, revolving on an axis in twenty-four hours; the land or solid matter composing it would be affected by the centrifugal force, like all other matter, but the cohesive principle which gives it solidity would prevent a derangement of its structure or change of position by such a cause, and the effect of the centrifugal force would therefore be confined to the fluid matter, which, in obedience to the tendency above described, would flow from either hemisphere toward the regions about the equator, where it would be gradually heaped up so as to form a convex protuberance around the line between the tropics, and to give to the earth, so far as the fluid matter upon it is concerned, the form of an oblate spheroid. But this movement of the fluid would cease as soon as the equatorial protuberance should attain a certain limit; for we may regard such a protuberance as a sort of mountain piled round the equator, down the sides of which there would be a tendency to fall, in obedience to gravitation, as would be the case down any other declivity.

The particles of fluid placed upon the side of this protuberance would be affected by two opposite forces: that which would result from the rotation

would have a tendency to move them toward the line-that is, ascending the acclivity-while their gravity, on the other hand, would have a tendency to make them descend, or to move them from the acclivity. When the protuberance would attain the limit at which these two tendencies would become equal, so that the descending force of gravity should be equal to the ascending force proceeding from the rotation, the particles of the fluid would be at rest, and would neither approach the line nor recede from it. It is within the province of mathematical physics to calculate what the limit of this protuberance would be which would produce this state of equilibrium, and the result of such calculations has given us a form which corresponds nearly to that which the earth is actually found to have.

But it may be objected that such reasoning would apply only to fluid matter upon the earth, whereas the oblate form is known to belong to its solid as well as its fluid surface.

This circumstance has been explained in two ways. 1. It is said that the earth in its original formation was altogether fluid; that in that fluid state it received its diurnal rotation, and consequently took the form corresponding with that rotation which we have just explained; that, by cooling down, the fluid matter partially hardened into a solid matter, leaving the liquid ocean corering about two thirds of the globe.

But if this original fluid state of the globe be denied or doubted, and if it be maintained that the globe received its revolution upon its axis when it was composed as it is, partly of land and partly of water, it is nevertheless contended that its present figure is explicable. If a true globe, diversified by land and by water, received a diurnal rotation like that of ours, the water would in the first instance flow toward the equator, and the geographical condition of the globe would be, two polar continents, separated by an extensive equatorial ocean. But after the lapse of ages, the ocean, washing continually upon the shores of the continents, would cause the constant abrasion of their solid matter, which, in the form of mud and sand, would mix with the liquid of the ocean, and would obey all its tendencies. In fact, in process of time the land by decadence and abrasion would obey the same principles which would affect a fluid; and the earth would at length, though after a long lapse of time, assume the form of fluid equilibrium. The present distribution of land and water which characterizes it has arisen from causes belonging more properly to geology than astronomy. Such is the theoretical reasoning applicable to the form of the earth. We are still, however, required by the rigorous principles of inductive philosophy to ascertain, as a matter of fact, independent of all theory, the actual figure of the globe. This has accordingly been done.

The section of an oblate spheroid made by a plane passing through the poles, is an oval, the longer axis of which is in the equator. It will be evident upon mere inspection that the curvature of the earth having such a form, would increase as we approach the equator, and diminish as we approach the poles; that is to say, a piece of a meridian taken near the equator would be part of a less circle than a similar piece taken near the poles. This is equivalent to stating that a degree of latitude near the equator would be shorter than a degree of latitude near the poles.

Thus, then, the question of the figure of the earth is in fact resolved into the measurement of a degree of latitude at different parts of the globe.

Such measurement has accordingly been executed with great precision, and it has been found, as was anticipated, that the degrees of latitude become shorter as we approach the equator, and longer as we approach the poles. A comparison of their lengths has given the degree that characterizes the oblateness of the earth.

But this is not the only test by which the figure of the earth has been ascertained. If the earth were a true globe revolving on its axis in twenty-four hours, the effect of its revolution would cause gravity to diminish on approaching the equator, and increase on approaching the poles; for the centrifugal force due to the rotation increasing toward the equator would cause a greater diminution of gravity there than toward the poles, where it lessens. Now, it is possible to calculate the effect of such centrifugal force upon the earth if it had the figure of a true globe. The effect of this diminution of gravity will be ascer tained with great exactness by observing the vibration of a pendulum in different parts of the earth. It has been already explained that the motion of a pendulum is produced by the gravity of the earth acting upon the ball of the pendulous body, and that the greater the attraction of gravity, the more rapid will be the vibration; and vice versa. We carry, then, a pendulum alternately toward the equator and toward the poles, and find invariably that its vibration is slower when taken toward the equator, and more rapid when taken toward the poles. But we find that this variation in its vibration does not correspond to that which it ought to have if the earth were an exact globe. It is just the variation which ought to take place if the earth were an oblate spheroid, of the form already described.

Thus we have two independent tests of the figure of the earth, which give accordant results.

LUNAR INFLUENCES.

The Red Moon.-Supposed Effect of the Moon on the Movement of Sap in Plants.-Prejudice respecting the time for felling Timber.-Extent of this Prejudice.-Its Prevalence among Transatlantic People. Prejudices respecting Effects on Grain.-On Wine.-On the Complexion.-On Putrefaction.-On Wounds.-On the Size of Oysters and Shellfish.-On the Marrow of Animals.— On the Weight of the Human Body.-On the Time of Births.-On the Hatching of Eggs.-On Human Maladies.-On Insanity.-On Fevers.-On Epidemics.-Case of Vallisnieri.-Case of Bacon.-On Cutaneous Diseases, Convulsions, Paralysis, Epilepsy, &c.-Observations of Dr. Olbers.