Yet the French have not been backward in like investigations. Understanding the value of commerce, their Dépôt de la Marine has not been inactive. Scandinavia has also done her part. The United States has accomplished one of the most thorough coast surveys ever undertaken by any nation; its difficulties are only to be measured by its extent. In fact, the whole civilised world has sent its messengers to the ends of the earth, and have united in this grand crusade of our age, the enriching of all men by a liberal system of interchange of the commodities of all climes.
COMPARATIVE GEOGRAPHY.
PART I.
THE SURFACE OF THE EARTH CONSIDERED IN ITS MOST GENERAL RELATIONS.
The Spheroidal Form of the Earth.
The measurements of, and investigations into the figure of the earth, have led, as already stated in the introduction, to no absolutely certain conclusion; yet they have made it certain that the earth is, in a general sense, a spheroid. There are many discrepancies, as were then stated, from the perfectly spheroidal shape; still it is in this sense a spheroid, that the polar diameter is not of the same length with the equatorial diameter.
The globular form of the earth, using that word in a loose sense, has been established with certainty since Newton’s time. The experience of circumnavigators, the uniform shield-shape of the shadow of the earth during eclipses of the moon, are witnesses to this. The gradual emerging and disappearance of objects, such as ships on the sea, in coming and going, caravans on the desert, of mountains as they are approached, establish the fact. These proofs are so well known that we but touch on them and pass to what is not so obvious.
As soon as the fact was established that the earth was a subordinate member of a system, it was brought into analogy with other planets, and their uniformly spherical shape was considered another valid reason for attributing the same to our globe. The discovery of the rotation of the earth on its axis was still another argument in the same direction. Mathematical measurements and observations of the pendulum, taken at different stations, have confirmed the same result.
To measure a spherical body, it is only necessary to take the length of a degree in one of its great circles, and to multiply its length by 360, the number of degrees. The method of measuring a degree on the earth’s surface is by taking two stars, just one degree apart, dropping, by astronomical and mathematical means, vertical lines upon the earth from them and measuring the distances apart of the points where those lines impinge upon the globe. This can be done with perfect accuracy. Such investigations show that the degrees are not all of equal length, as they would be were the earth a perfect sphere. Nearer the poles they are longer, nearer the equator they are shorter. The curvature of the earth is therefore greater as you approach the equatorial line, and less as you recede from it. In general terms, then, the earth is an oblate spheroid, as it would be a prolate spheroid were the lengths of its diameters reversed. By the most accurate measurements, those of the astronomer Bessel, if the equatorial diameter were divided into 289 equal parts, the polar diameter would measure 288 of them, being ¹⁄₂₈₉ shorter.