The two stations, of Beachy Head in Sussex and Dunnose in the Isle of Wight, are visible from each other, and more than 64 miles asunder, nearly in a direction from east to west; their exact distance was found by the geodetical operations to be 339,397 feet (64 miles and 1477 feet). The azimuth, or bearing of the line between them with respect to the meridian, and also the latitude of Beachy Head, were determined by astronomical observations. From these data the length of a degree perpendicular to the meridian was computed; and this, compared with the length of a meridional degree in the same latitude, gave the proportion of the polar to the equatorial axis. The result thus obtained, however, differed considerably from that obtained by meridional degrees. It has been found impossible to explain the want of agreement in a satisfactory way. * * By comparing the celestial with the terrestrial arcs, the length of degrees in various parallels was determined as in the following table:—
| Latitude of middle point. | Fathoms. | |||
|---|---|---|---|---|
| ° | ′ | ″ | ||
| Arbury Hill and Clifton | 52 | 50 | 29·8 | 60,766 |
| Blenheim and Clifton | 52 | 38 | 56·1 | 60,769 |
| Greenwich and Clifton | 52 | 28 | 5·7 | 60,794 |
| Dunnose and Clifton | 52 | 2 | 19·8 | 60,820 |
| Arbury Hill and Greenwich | 51 | 51 | 4·1 | 60,849 |
| Dunnose and Arbury Hill | 51 | 35 | 18·2 | 60,864 |
| Blenheim and Dunnose | 51 | 13 | 18·2 | 60,890 |
| Dunnose and Greenwich | 51 | 2 | 54·2 | 60,884 |
This table presents a singular deviation from the common rule; for instead of the degrees increasing as we proceed from north to south, they appear to decrease, as if the Earth were an oblong instead of an oblate spheroid. * * The measurements of small arcs of the meridian in other countries have presented similar instances.”[4]
[4] Encyclopedia of Geography, by Hugh Murray and several Professors in the University of Edinburgh.
A number of French Academicians who measured above three degrees of the meridian in Peru, gave as the result of their labours the first degree of the meridian from the equator as 56,653 toises; whilst another company of Academicians, who proceeded to Bothnia in Lapland, gave as the result of their calculation 57,422 toises for the length of a degree cutting the polar circle. But a more recent measurement made by the Swedish Astronomers in Bothnia shows the French to have been incorrect, having given the degree there 196 toises more than the true length. Other observations have been made, but as no two sets of experiments agree in result, it would be very unsatisfactory to conclude from them that the Earth is an oblate spheroid.
Returning to the pendulum, it will be found to be equally unsatisfactory as a proof of this peculiar rotundity of the Earth. It is argued that as the length of a seconds pendulum at the equator is 39,027 inches, and 39,197 inches at the north pole, that the Earth must be a globe, having a less diameter through its axis than through its equator. But this proceeds upon the assumption that the Earth is a globe having a “centre of attraction of gravitation,” towards which all bodies gravitate or fall; and as the pendulum is a falling body under certain restraint, the fact that it oscillates or falls more rapidly at the north than it does at the equator, is a proof that the north is nearer to the centre of attraction, or the centre of the Earth, than is the equatorial region; and, of course, if nearer, the radius must be shorter; and therefore the “Earth is a spheroid flattened at the poles.” This is very ingenious and very plausible, but, unfortunately for its character as an argument, the essential evidence is wanting that the Earth is a globe at all! whether oblate or oblong, or truly spherical, are questions logically misplaced. It should also be first proved that no other cause could operate besides greater proximity to the centre of gravity, to produce the variable oscillations of a pendulum. This not being attempted, the whole subject must be condemned as logically insufficient, irregular, and worthless for its intended purpose. Many philosophers have ascribed the alterations in the oscillations of a pendulum to the diminished temperature of the northern centre. That the heat gradually and almost uniformly diminishes on passing from the equator to the north is well ascertained. “The mean annual temperature of the whole Earth at the level of the sea is 50° Fah. For different latitudes it is as under:—
| Degrees. | Inches. | |||||||
|---|---|---|---|---|---|---|---|---|
| Latitude | (Equator) | 0 | 84·2 | Length | of | Pendulum | 39,027 | |
| „ | „ | 10 | 82·6 | „ | „ | „ | ||
| „ | „ | 20 | 78·1 | „ | „ | „ | ||
| „ | „ | 30 | 71·1 | „ | „ | „ | ||
| „ | „ | 40 | 62·6 | „ | „ | „ | ||
| „ | (London) | 50 | 53·6 | „ | „ | 39,139 | ||
| „ | „ | 60 | 45·0 | „ | „ | „ | ||
| „ | „ | 70 | 38·1 | „ | „ | „ | ||
| „ | „ | 80 | 33·6 | „ | „ | „ | ||
| „ | (Pole) | 90 | 00·0 | „ | „ | 39,197 | [5]” | |
[5] “Million of Facts,” by Sir Richard Phillips, p. 475.
“All the solid bodies with which we are surrounded are constantly undergoing changes of bulk corresponding to the variations of temperature. * * The expansion and contraction of metals by heat and cold form subjects of serious and careful attention to chronometer makers, as will appear by the following statements:—The length of the pendulum vibrating seconds, in vacuo, in the latitude of London (51° 31′ 8″ north), at the level of the sea, and at the temperature of 62°, has been ascertained with the greatest precision to be 39·13929 inches: now, as the metal of which it is composed is constantly subject to variation of temperature, it cannot but happen that its length is constantly varying; and when it is further stated that if the “bob” be let down ¹⁄₁₀₀th of an inch, the clock will lose 10 seconds in 24 hours; that the elongation of ¹⁄₁₀₀₀th of an inch will cause it to lose one second per day; and that a change of temperature equal to 30° Fah. will alter its length ¹⁄₅₀₀₀th part and occasion an error in the rate of going of 8 seconds per day, it will appear evident that some plan must be devised for obviating so serious an inconvenience.”[6]
[6] “Noad’s Lectures on Chemistry,” p. 41.