. every square mile of the surface of Great Britain, would, at a medium, be 1,944,633, or nearly two millions of tons. This gives about three thousand tons of water for each English acre, à quantity equal to 630,000 imperial gallons.” The contributions of the several months to the production of this quantity, are stated by Mr. Daniell, and recorded in the British Almanac, to be in the following proportions : January 1.483 2.516 0.746 1.453 1.440 2.193 1.786 2.073 1.853 2.400 1.830 2.426 The greatest average quantity, therefore, falls in July, and the smallest in February In comparing quantities which fall in the twenty-four hours, which constitute a day, the result of experiment shows that a greater amount of rain falls while the sun is below, than when above the horizon. One very remarkable circumstance attending the fall of rain, is, “ that smaller quantities have been observed to be deposited in high than in low situations, even though the difference of altitude should be inconsiderable. Similar observations have been made at the summit, and near the base of hills of no great elevation. Rain-gauges, placed on both sides of a hill at the bottom, always indicate a greater fall of rain than on the exposed top."* If the whole of the waters which fall from the heavens were to return again, the evaporation from the ground might be sufficient alone to inaintain the perpetual circulation. But more than one-third of all the rains and snows are carried by the rivers into the ocean, which must hence restore this continued waste. * * Leslie. *** The Natural History of the Weather embraces the phenomena of Dew and Hoar Frost, Dryness and Moisture, Heat and Cold, Thunder and Lightning, and Winds : as well as those of Evaporation, Clouds, Fogs and Mist, and Rain, to which the present article is necessarily limited by our space. The subject will be pursued in the Almanac for 1832. II. SHAPE OF THE EARTH, AND ITS SIZE. The level portions of the earth's surface seem at first view perfectly flat. But if we examine them more critically, and for a considerable extent, we shall find that they are decidedly convex, or swelled out in the middle. The light of a light-house requires to be raised, in order to be seen at any considerable distance. Let it be placed on a level with the sea, and a person of the common height, or whose eyes are less than six feet above the surface of the sea, would not be able to see it at the distance of four miles, however strong and clear the light might be. But upon raising himself higher and higher, he would at length, when his eye had reached an elevation of ten or eleven feet above the surface, be able to discern it just grazing the surface of the water. The same effect would be produced if the light were raised ten or eleven feet, and the eye of the observer were on the level of the ocean. And a light 60 or 100 feet high disappears in like manner by sinking lower and lower; only the distance at which we are required to place ourselves to produce this effect, becomes greater and greater according to the elevation of the light, and according also to our own elevation above the level of the sea. The most convenient position for a nice observation of this kind is an extended lake, when covered with smooth ice. We will suppose ourselves provided with a common leveling instrument, or any long tube capable of being fixed in an exactly horizontal position, which is easily determined by a water-level, or by being at right angles to a plumb-line. Let us suppose that the line of sight through the tube is precisely four feet from the ice, and that the tube can be turned in all directions without varying from a horizontal or level position. If we now look through the tube at an upright rod or pole placed with one end on the ice at different distances, we shall be able to establish, in the most satisfactory manner, the following important facts. 1. The line of sight, or apparent level, as it is called, departs from the surface of the ice, or true level, in whatever direction we look. 2 This departure, or difference of level, is the same in all directions as to the points of the compass, where the distance from the observer is the same. 3. The difference of level for a distance of one mile is 8 inches. 4. If we double any distance, the difference of level is quadrupled, and if we triple the distance, the difference of level is nine times as great, and so on, according to the law of the squares ; that is, the difference of level for one mile being 8 inches, that for two miles is not twice 8, but four times 8, or 32 inches, and that for three miles is 9 times 8, or 72 inches. Similar observations being made in other places in different parts of the earth, we arrive at essentially the same results. The facts above given, lead to conclusions not less curious and striking. 1. The earth's surface is curved instead of being plane, or flat, and plumblines or lines perpendicular to the surface, are not strictly parallel, but incline more and more the further they are apart, and tend to meet at some point within. 2. The earth appears to be equally curved in all directions, and the law of the departure of the apparent from the true level, indicates a spherical surface. 3. The particular departure of 8 inches to a mile points out the dimensions of the earth, and furnishes, by means of a simple proposition in geometry, a method of calculating its diameter. Thus A B in the adjoining figure we have AB and BD to find AE, or BE, which does not sensibly differ from AE, since BD, by supposition, is only eight inches. It is a very familiar proposition in plane geometry, that, when from a point without a circle two lines be drawn, one cutting and the other touching it, the touching line is a mean proportional between the cutting line and E the part without the circle; hence BD: AB :: AB: BE or AE very nearly ; that is, 8 inches being 7927 of a mile, 7020:1::1: 7920; in other words, the earth's diameter is 7920 miles. This is almost precisely what it is fixed at by the most elaborate observations and calculations. As the circumference of a circle or sphere exceeds its diameter a little more than three times (37), if we multiply the above result by 37, we have the circumference equal to 24,890 miles. The common way of determining the magnitude of the earth, is by measuring a certain part of its circumference in the direction of the meridian. Lake Champlain, for example, when frozen over, would furnish a proper field for such an operation. Two plumb-lines being suspended, on the same rneridian, one at Crown Point and the other on the boundary lime between the United States and Canada, would be found to deviate from parallelism one degree, that is, they would meet near the centre of the earth, having an inclination, or forming an angle, of one degree, or ū part of a circumference, and the distance between these plumblines being actually measured with a chain, would be the 360th part of the entire circuit of the globe. The inclination of the plumb-lines above mentioned, is the same thing as the difference of latitude of the two places, and is found by taking the altitude (or angular distance above the horizon) of the pole by means of the Pole star, or other star in the neighborhood. Portions of the earth's circumference, in various countries and regions, have been determined in this way with the greatest care and exactness, and the final result of all these operations is very nearly what we have stated above. But while we have thus found out the general dimensions of the earth, we have discovered that the form is not exactly that of a sphere. The length of a degree increases as we proceed from the equator toward either pole. We hence infer that the earth is flattened about the polar regions, and more convex between the tropics. The average length of a degree is 6915 miles. But the length of a degree in latitude 66°, is about two-thirds of a mile greater than at the equator. The same phenomenon is indicated also by the pendulum. A clock which keeps correct time at the equator, is found to gain more and more as it is carried toward either pole in consequence of a quicker motion of the pendulum, resulting from a nearer approach to the centre, and a greater power of gravity. The results of calculations founded upon observations of the pendulum agree pretty nearly with those derived from actual measurement; and the conclusion from the whole is, that the degree of fattening amounts to about sio of the whole diameter of the earth, that is, a line drawn through the centre of the earth, from pole to pole, is zio (or 26 miles) shorter than a similar line in the direction of the equator. III. DENSITY OF THE EARTH. Is the earth solid or hollow, and if solid, how dense is it? Would it be equivalent to so much water, or would it exceed it, and how much would it exceed it? It may seem very difficult to answer these questions, and yet they have been answered most satisfactorily. It is now abundantly proved not only that the earth is solid, but that the interior parts are more and more compact the nearer we approach to the centre, as we should naturally suppose. We are able to estimate the influence which a mountain exerts upon a plumb-line by observing how much it is drawn out of the direction of an exact perpendicular ; and then, by comparing the size of the mountain with the size of the earth, knowing at the same time of what materials the mountain is composed, we are able to say how much the matter of the whole earth exceeds that of the mountain. It is thus ascertained that the matter composing the earth is about five times as dense as water, or, in other words, would weigh, under the same circumstances, five times as much as the same bulk of water. Now we know that the matter near the surface, is, for the most part, either water or earthy and stony substances, only two or three times as heavy as water. The density of the interior parts, therefore, must greatly exceed that at the surface, in order that the average may amount to five times the density of water, as is ascertained by actual observation. It may be thought, that the above method of determining the quantity of matter in a mountain is liable to great uncertainty. It should be 104 TEMPERATURE OF THE INTERIOR OF THE EARTH. known that we do not rely upon a single experiment, or even upon one single method, for so important a result. A balance has been contrived, depending upon the twisting and untwisting of an extremely fine wire suspended perpendicularly,* by which the mutual tendency (or relative weight) of two balls of lead, has been accurately estimated and compared with the force exerted by the great mass of the earth; and these delicate experiments have afforded a striking confirmation of the result above stated. IV. TEMPERATURE OF THE INTERIOR OF THE EARTH. The circumstance of the earth's being flattened at the poles and protuberant at the equator, is the natural and necessary result of its rotation on its axis. But in order that it might yield to the force resulting from such a motion, the matter of which it is composed, must have been soft. Now, although water is capable of being compressed, and so far as we can judge, of taking any degree of density, according to the force exerted upon it, still the shape of the earth is not that which would have resulted from such a mass of water. There may be particular portions of the sea that extend to the depth of several miles, as there are particular points of the solid crust of continents, that rise to this height above the general level. Still we have reason to believe, that the average depth of the ocean does not much exceed three thousand feet. It is thought that heat may have been the original cause of the fluidity of the earth, and that there may still remain enough to keep the interior portions in the same state. The more this subject has been examined, the more the evidence has accumulated in favor of the position that the temperature increases as we descend below the surface. There are numerous instances in which we have been able, by means of natural or artificial excavations, to penetrate to the depth of from 1300 to 1600 feet. The general inference from all the observations made in different parts of the earth is, that there is an increase of heat amounting to about 1° of Fahrenheit for every 46 feet in depth; that at the depth of 10,000 feet the heat would be sufficient to boil water, and that at the depth of about 100 miles, or do part of the distance to the centre, the heat would be intense enough to melt most of the earths and stones that are known to enter into the composition of the globe. These facts and inferences have an important bearing upon the phenomena of earthquakes and volcanoes, and open a wide field of speculation to the natural historian and geologist. * A balance of this construction, applied to electrical forces, has been estimated to weigh to the sixty-thousandth part of a grain. |