or to the thermometers which exhibited the temperature of the surrounding air. It was intended also to interchange the iron stands, but when once fixed in position, their stability was so great that it was considered wiser not to interfere with them during the experiments.

These scientific operations were carried on whilst the labourers at the mine were in full work. Indeed, it was no unusual circumstance to see several of these hardy pitmen resign their position at the mouth of the shaft, to allow the "philosophers" to descend in the cage to their mysterious chamber, discussing, meanwhile, many different opinions on the object of this curious enterprise. During the three weeks occupied by the experiment, nothing untoward occurred to interrupt the observations; but on the day after the instruments had been removed, an accident occurred in the shaft to some of the lifting apparatus, which, had it happened in the course of the experiments, would have caused considerable delay, perhaps injuriously affecting the result.

It has been already stated that the direct object of these experiments was to ascertain the amount of the daily acceleration of a seconds' pendulum at the bottom of a mine, over another placed in similar circumstances on the surface; and to assume that any increased velocity attained at the lower station was caused by a greater force of gravity acting on the pendulum. Now, it was found from these experiments that this force of gravity was greater at the lower station than at the upper, by about 1000 part of the whole force, and that its effect on the pendulum was such as to cause that at the lower station to make two and a quarter vibrations a day more than the corresponding one at the upper station.

1

Several other important considerations had to be taken into account before the final result could be obtained, giving the mean or average density of the Earth. For example, the formation of the crust or shell between the two stations had to be completely investigated; the surface of the neighbouring country was surveyed; and the thickness of every stratum of earth, rock, coal, etc., was measured in the shaft. One hundred and forty-two different layers were found. Of these, the specific gravity of the principal strata was determined by Professor Miller, of Cambridge. From these specimens, the average density of the Earth's crust at Harton is about two and a half times greater than water.

After combining the amount of daily acceleration of the lower pendulum, with the specific gravity, etc., of the Earth's surface, according to mathematical principles far too abstruse for these pages, the globe on which we dwell was found to have a mean density of six and a half times greater than that of water. This result is somewhat larger than those obtained from former experiments; but the Astronomer Royal considers that "it is entitled to compete with the others, on at least equal terms."

From these researches, it is not difficult to determine the actual weight of the Earth in pounds avoirdupois, knowing as we do the exact size and volume of our globe; but it is not the Earth only which can be weighed in this manner, for, using its mass and weight as the units of measurement, the masses and weights of the Sun and all the planets of the solar system can be inferred. That the knowledge of these

THE EARTH.

masses is invaluable, it is only necessary to say, that the accurate predictions of the positions and motions of the heavenly bodies could never be performed without reliable data concerning the power of attraction which one body has on another, which power is regulated in a considerable degree by their masses.

Sometimes in the evenings of spring, near the equinox, a luminosity in the form of a cone may be observed shooting upwards from the horizon where the Sun had set, reaching a considerable distance towards the zenith. It has been frequently taken by those who are not familiar with the ordinary aspect of the sky, for the Milky Way, for an aurora, or the remains of twilight. It is generally known, however, by the name of the Zodiacal Light. It is observed much more favourably in tropical countries, where it has been noticed from day to day, and its position, with respect to the stars, recorded with the greatest minuteness. What this celestial phenomenon is composed of has only been a matter of conjecture. It has been suggested that it may probably proceed from the reflected light of myriads of meteors which are now known to be traversing space even to the extreme limits of the solar system. Humboldt, however, considers that it may possibly be a vast nebulous ring, rotating between the Earth and Mars, or, less probably, the exterior stratum of the solar atmosphere. He thus remarks on its appearance in the equatorial heavens:-"Those who have lived for many years in the zone of palms must retain a pleasing impression of the soft beauty with which the zodiacal light, shooting pyramidally upwards, illumines a part of the uniform length of tropical nights. I have seen it shine with an intensity of light equal to the Milky Way in Sagittarius, and that not only in the rare and dry atmosphere of the summits of the Andes at an elevation of from thirteen to fifteen thousand feet, but even on the boundless grassy plains, the Llanos of Venezuela, and on the seashore, beneath the very clear sky of Cumana. This phenomenon was often rendered especially beautiful by the passage of light fleecy clouds, which stood out in picturesque and bold relief from the luminous background. In our gloomy so-called 'temperate' northern zone, the zodiacal light is only distinctly visible in the beginning of spring, after the evening twilight, in the western part of the sky, and at the close of autumn, before the dawn of day, above the eastern horizon."

Other travellers have noticed the superior brilliancy of the zodiacal light when viewed from stations situated within the tropics. Major Tennant, R.E., made a series of observations of its position among the stars from the Bay of Bengal, during a voyage to Calcutta in the early part of 1868. He frequently watched it, at intervals from sunset until nine in the evening; and after the glow of sunset was gone he always found the shape to be a portion of a long ellipse, or parabola, ill-defined at the outlines, but towards the axis and horizon there was a marked condensation of light. M. Du Chaillu, in his African wanderings, was also much impressed with the beauty of this phenomenon. When occasionally enchanted by the superb appearance of the heavens, at a time when most of the constellations to which we have previously alluded have been only a few degrees south of the zenith, the zodiacal light has

appeared to add fresh interest to the scene. Referring to the magnificent spectacle, he remarks:-"Then, as if to give a still grander view to the almost enchanting scene, the zodiacal light rose after the Sun had set, increasing in brilliancy, of a bright yellow colour, and rising in a pyramidal shape high into the sky, often so bright that it overshadowed the brightness of the Milky Way and the rays of the Moon, the beautiful yellow light gradually diminishing towards the apex. It cast a gentle radiance on the clouds round it, and sometimes formed almost a ring, but never perfect, having a break near the meridian; at times being reflected in the east with nearly as much brilliancy, if not as much, as in the west, and making me almost imagine a second sunrise. April and May were the months when the light showed itself in its greatest brilliancy. On April 13th, 1865, the glow coming from the west was so bright that it totally hid the Milky Way in the principal part of its course. I could only distinguish it above the sword of Orion; the glow was the brightest below the planet Mars, and the base of the pyramid reached, on the south, the part of the Milky Way, at the foot of the Cross." The zodiacal light, as observed in England, can bear no comparison in intensity with the appearances above, probably owing to the greater amount of twilight north of the tropical zone. It has, however, been seen in this country, very favourably on some occasions; at times we have been able to notice and record its relative position with respect to neighbouring celestial objects with great exactness. To an observant eye it is sometimes distinguishable in the spring evenings without much effort. But the equatorial skies alone must be scanned to see with advantage the pyramidal form and the superior lustre of this curious phenomenon.

M. Borelly, of the Marseilles Observatory, observed, on the evening of the 30th of January, 1869, a very splendid exhibition of the zodiacal light. While searching for comets in the western part of the sky, the field of his telescope was lightened up with it, so that the small stars were very difficult to see. The light mounted upwards from that part of the horizon near which the Sun had set.

MARS.

Mars is the fourth planet from the Sun, being the next in order from the Earth. Its surface exhibits a greater analogy to that of our own globe than that of any other planet of the solar system; and when it is at its least distance from us, it shines with great splendour in the midnight sky. Mars can always be distinguished from the other planets, and from the fixed stars, by its ruddy light. Owing to its occasional near approach to our Earth, great facilities are obtained for delineating the various lights and shades on its disk; and at such opportunities numerous accurate drawings are always made. When viewed through large telescopes, the surface of Mars represents the outline of continents and seas, and near the poles white spots are clearly visible, which owe their existence probably to an accumulation of snow in the polar regions.

MARS.

The mean distance of Mars from the Sun is one hundred and forty millions of miles, and its nearest distance from the Earth averages forty-eight millions. The equatorial diameter of Mars is about 4,363 miles, and its polar diameter about seventy miles less. Mars revolves around the Sun in a few minutes less than 687 days. When in or near opposition, the form of Mars is sensibly globular, but in other portions of its orbit it is generally seen gibbous.

The apparent magnitude of Mars is very variable. When on the opposite side of the Sun with respect to the Earth, and at its greatest distance from us, its telescopic diameter amounts to less than four seconds of arc; but at the times of its nearest approach at favourable oppositions, its telescopic diameter is increased to twenty-four seconds of arc. When viewed on these occasions with a good telescope, the disk of Mars appears covered with various markings of a very distinct character, some of which give those defined appearances of continents and seas which have been so ably depicted by so many astronomers. The brightest parts, excepting the white patches near the pole, have a ruddy tint, while the darker portions have a greenish hue, the effect probably of contrast. It is by the observation, at different epochs, of the positions of these fixed markings on the disk of Mars, that the most accurate determinations of its rotation have been made. One recently published by Mr. Proctor gives the value 24h 37m 22-73", which is probably true to the hundredth part of a second.

If there be any inhabitants on Venus, the surface of the Earth will appear to them very similar to that exhibited to us in telescopic views of Mars. They will, however, have the advantage of seeing a globe of larger dimensions, but the general aspect of the terrestrial surface, including the distribution of land and water, will be the same. The poles of the Earth would probably appear comparatively bright, as in Mars, if it be true that our unexplored polar regions are covered with ice and snow. Although the atmosphere of Mars is not considered to be so dense as was formerly attributed to that planet, yet it is proved that, like the Earth, it is surrounded by one of sufficient density to obscure occasionally the various markings on its surface, particularly those near the edges of the planet. Mars is doubtless subject to similar meteorological phenomena as our Earth, and to perhaps greater sudden changes of weather. Professor Phillips, of Oxford, has remarked, that the great interchange of the humidity of the atmosphere which must necessarily take place periodically between the two hemispheres, and especially between the two poles, would give rise to very violent hurricanes, of which we have little conception. The difference in the lengths of the years of the Earth and Mars, is one point where the analogy between the two planets fails, for as the Martial year is nearly twice as long as that of the Earth, the seasons on Mars will be lengthened in a corresponding proportion. For example, in the northern hemisphere of Mars, spring lasts 191 Martial days, summer 181 days, autumn 149 days, and winter 147 days; and as the seasons are reversed in the southern hemisphere, spring and summer, taken together, last seventy-six days longer in the northern hemisphere than in the southern.

Mars is the only large planet, exterior to the Earth, without a satellite. Being at a greater distance from the Sun than our globe, one would suppose that there would be a greater necessity for an attendant Moon than with us, but no telescopic aid has been able to detect any object near the planet. If, however, a very small Moon were situated comparatively near to the surface of Mars, our present optical means would probably not be sufficiently powerful to perceive it. In the absence of a satellite, the nights of Mars must be always obscured, relieved only by occasional auroral displays, or by the morning and evening twilight before and after the Sun is above the horizon. The mass of Mars is only one-eighth part of that of the Earth. Its orbital velocity, or motion in space around the Sun, is 53,514 miles per hour; and the velocity of its rotation at the equator 558 miles per hour.

Until recent years the ruddy tint of Mars was universally believed to owe its origin to an unusually dense atmosphere; but this opinion has been considerably modified since the surface of the planet has been so carefully examined by Professor Phillips, Mr. De La Rue, Mr. Lockyer, Mr. Huggins, and others. It has been found that the light reflected from the neighbourhood of the poles has no trace of colour, although in its course it has passed through a denser atmosphere than that which is found on the central portions of the disk, where the ruddy tint is most apparent. Mr. Huggins, who has made some very important observations of the spectrum of the solar light reflected from Mars, remarks that "if indeed the colour be produced by the planet's atmosphere, it must be referred to peculiar conditions of it which exist only in connection with particular portions of the planetary surface. The evidence we possess at present appears to support the opinion that the planet's distinctive colour has its origin in the material of which some parts of its surface are composed. Mr. Lockyer's observation, that the colour is most intense when the planet's atmosphere is free from clouds, obviously admits of an interpretation in accordance with this view."