is accompanied by a satellite. If Venus were attended by a small body in close proximity, it would be conceivable that under ordinary circumstances the brilliancy of the planet would obliterate the feeble beam of rays from the minute companion, and thus the satellite would remain undiscovered. It was therefore a matter of great interest to scrutinise the vicinity of the planet while in the act of transit over the sun. If a satellite existed-and the existence of a satellite has often been suspected-then it would be capable of detection against the brilliant background of the sun. Special attention was directed to this point during the recent transits, but no satellite of Venus was to be found. It seems, therefore, to be very unlikely that Venus can be attended by any satellite of appreciable dimensions.

The observations directed to the investigation of the atmosphere surrounding Venus have been more successful. If the planet were devoid of an atmosphere, then it would be totally invisible just before commencing to enter on the sun, and would relapse into total invisibility as soon as it had left the sun. The observations made during the transits are not in conformity with such suppositions. Special attention has been directed to this point during the recent transits. The result has been very remarkable, and has proved in the most conclusive manner the existence of an atmosphere around Venus. As the planet gradually moved off the sun the circular edge of the planet extending out into the darkness was seen to be bounded by a circular arc of light, and Dr. Copeland, who observed this transit under exceptionally favourable circumstances, was actually able to follow the planet until it had passed entirely away from the sun, at which time the globe, though itself invisible, was distinctly marked by the girdle of light by which it was surrounded. This luminous circle is inexplicable save by the supposition that the globe of Venus is surrounded by an atmospheric shell in the same way as the earth.

It may be asked, what is the advantage of devoting so much time and labour to a celestial phenomenon like the transit of Venus which has so little bearing on practical affairs? What does it matter whether the sun be 95,000,000 miles off, or whether it be

only 93,000,000, or any other distance? We must admit at once that the inquiry has but a slender bearing on matters of practical utility. No doubt a fanciful person might contend, that to compute our Nautical Almanacs with perfect accuracy we require to know the distance of the sun with accuracy. Our mighty commerce depends on skilful navigation, and one factor in successful navigation is the reliability of the Nautical Almanac. The increased perfection of the almanac must therefore bear some relation to increased perfection in navigation. Now, as good authorities tell us that in running for a harbour on a tempestuous night, or in other critical emergencies, even a yard of sea-room is often of great consequence, so it may conceivably happen that to the infinitesimal influence of the transit of Venus on the Nautical Almanac is due the safety of a gallant vessel.

But the time, the labour, and the money expended in observing the transit of Venus are really to be defended on quite different grounds. We see in it a fruitful source of information. It tells us the distance of the sun, which is the foundation of all the great measurements of the universe. It gratifies the intellectual curiosity of man by a view of the true dimensions of the majestic solar system, in which the earth is seen to play a dignified, though still subordinate, part; and it leads us to a conception of the stupendous scale on which the great universe is constructed.

It is not possible for us, with a due regard to the limits of this volume, to linger any longer over the consideration of the transit of Venus. When we begin to study the details of the observations, we are immediately confronted with a multitude of technical and intricate matters. On the occasion of a transit, it has first to be decided where the observations are to be made-in itself a question that has led to no little discussion. Then the instruments that are to be used, and the description of observations to be made, have to be investigated with considerable complexity. The observers must be specially trained for the work, for even Methuselah himself could hardly have lived long enough to have had much practice in the observations of transits of Venus. To compensate for the inevitable want of experience, the observers had to be pre

pared by a special course of instruction, in which a fictitious transit was observed. Then, too, the interpretation of the observations involves many thorny and many controverted questions. To pursue all these matters so as to render them intelligible would lead us into great detail, and therefore we do not make the attempt. This course is the more advisable when it is remembered that the transit of Venus is only one of the methods of finding the sun's distance— a celebrated one, no doubt, but not perhaps the most reliable. It seems not unlikely that the final determination of the sun's distance will be obtained in quite a different manner. This will be explained in Chapter XI., and hence we feel the less reluctance in passing away from the consideration of the transit of Venus as a method of celestial surveying.

We must now close our description of this lovely planet; but before doing so, let us add—or in some cases repeat-a few statistical facts as to the size and the dimensions of the planet and its orbit.

The diameter of Venus is about 7,660 miles, and the planet shows no measurable departure from the globular form, though we can hardly doubt that its polar diameter must really be somewhat shorter than the equatorial diameter. This diameter is only about 258 miles less than that of the earth. The mass of Venus is about three-quarters of the mass of the earth; or if, as is more usual, we compare the mass of Venus with the sun, it is to be represented by the fraction 1 divided by 425,000. It is to be observed that the mass of Venus is not quite so great in comparison with its bulk as might have been expected. The density of Venus is about 0.850 of that of the earth. Venus would weigh 4.81 times as much as a globe of water of equal size. The gravitation at the surface of Venus will, to a slight extent, be less than the gravitation at the surface of the earth. A body here falls sixteen feet in a second; a body let fall at the surface of Venus would fall about three feet less. The time of rotation of Venus is an element about which there is still considerable uncertainty. It is supposed to occupy about twenty-three hours and twenty-one minutes.

L

The orbit of Venus is remarkable for the close approach which it makes to a circle. The greatest distance of Venus from the sun does not exceed by 1 per cent. the least distance. Its mean distance from the sun is about 67,000,000 miles, and the movement in the orbit has a mean velocity of nearly 22 miles per second, the entire journey being accomplished in 224-70 days.

CHAPTER IX.

THE EARTH.

The Earth is a great Globe-How the Size of the Earth is Measured-The Base Line The Latitude found by the Elevation of the Pole-A Degree of the Meridian-The Earth not a Sphere-The Pendulum Experiment-Is the Motion of the Earth slow or fast?-Coincidence of the Axis of Rotation and the Axis of Figure-The Existence of Heat in the Earth-The Earth once in a Soft Condition-Effects of Centrifugal Force-Comparison with the Sun and Jupiter --The Protuberance at the Equator-The Weighing of the Earth-Comparison between the Weight of the Earth and an equal Globe of Water-Comparison of the Earth with a Leaden Globe-The Pendulum-Use of the Pendulum in Measuring the Intensity of Gravitation—The Principle of Isochronism-Shape of the Earth Measured by the Pendulum.

THAT the earth is a round body is a truth immediately suggested to us by the most simple astronomical considerations. The sun is round, the moon is round, telescopes show that the planets round. No doubt comets are not round, but then a comet is not a solid body at all. We can see right through a comet, and its weight is too small for our measures to appreciate. If, then, all the solid bodies we can see are round globes, is it not likely that the earth is a globe also? But we have far more direct information than mere surmise.,

There is no better way of actually seeing that the surface of the sea is curved than by watching on a clear day a distant ship. When the ship is a long way off and is still receding, its hull will gradually disappear, while the masts will remain visible. On a fine summer's day we can often see with, or indeed without an opera-glass, the top of the funnel of a steamer appearing above the sea, while the body of the steamer is below. If the sea were perfectly flat, there is nothing to obscure the body of the vessel, and it therefore would be visible as long as the funnel remains visible; but if the sea be really curved the pro