תמונות בעמוד
PDF
ePub

of the earth, and to enjoy the splendour of its reflected light. By observing the rotation of the earth on its axis, which may casily be done from the regular appearance and disappearance of the darker and brighter spots on its surface, the inhabitants of the moon are provided with a convenient and accurate mode of measuring their time, and dividing their own day equal to one of our months, into small portions equal to one twenty-ninth or one thirtieth part of their day.

The conjectures of ingenious men respecting the nature of the moon, and particularly of her surface, have been various and opposite : but the opinion now generally enter"tained is that the dark parts which we observe are water, or at least some substance of that nature, because water we know absorbs instead of reflecting the rays of light; but in these dark parts good glasses have discovered hollows and pits of various shapes and magnitudes, preserving an uniform position and appearance, which could not be the case did the dark parts consist of a fluid like water. Other observers however have suggested that in these bodies of water, or similar fluid, islands may exist, containing pits and hollows visible to us. That the moon however posSesses many mountains, of considerable elevation in proportion to her diameter, seems to be beyond a doubt. If, with a good telescope, we observe her surface when three, four, or five days old, we perceive, besides the entirely illu. minated portion, a number of bright spots or points situated within the immediately adjoining dark part. Continuing our observation for some time, we discover those bright spots gradually to increase in size, and at last the space where they were situated becomes wholly enlightened, uniting with the part at first enjoying the sun's rays, whilst fresh bright spots continue to present themselves in regular succession, as the moon in her course round us shews 2 G 2

the

the several parts of her surface to the sun. These appearances are similar and analogous to what is observed on the earth; when the sun first rises upon our horizon, the tops of the highest mountains are the first gilded by his rays ; as the earth performs her rotation on her axis, these points turning gradually more and more towards the sun, his light falls lower down the mountain sides, until the whole, with the vallies at their feet, are fully enlightened, at which time lofty summits, hitherto in darkness, begin in a similar manner to receive his rays, and thus by regular progression come to be wholly illuminated like their predecessors. Astronomers, proceeding on erroneous principles, had estimated the mountains of the moon to be much more elevated than those of the earth, some being supposed to exceed nine English miles in height, nearly three times that of the highest of our summits; but later and more accurate observations make it evident that the elevation of the lunar mountains has been greatly over-rated, and that, with a very few exceptions, their general height does not exceed one-half of an English mile, while the highest summit, hitherto measured, rises to about a mile and a half. From certain peculiarities in the appearance of the lunar mountains it is probable that some of them are volcanoes. Herschel, in 1787, observed one of these in a state of eruption, the diameter of the burning part being not less than three miles : the appearance of the actual fire or volcanic eruption he compared to a small piece of burning charcoal, covered by a very thin coat of white ashes, as frequently happens when it has been some time burning : This fire had a degree of brightness about as strong as that with which such a piece of coal would be seen to glow in faint day-light; all the adjacent parts of the vol. canic mountain seemed to be illuminated by the eruption, in a greater or less degree as they lay nearer to or more remote from the volcano.

rays; but

Whether years attraction

[ocr errors]

Whether the moon has any fine fluid enclosing her body, similar to our air, has long been a subject of discussion among astronomers; but whether she has or has not any atmosphere, it is now generally allowed that it is to our senses almost entirely insensible.

That the moon has a very important influence on this earth, in various respects, has long been a received opinion : not only the state of the weather, it is supposed, in a great measure depends on or may be foretold by her constantly changing appearances ; but even the human mind is so far subjected to her power, that those unhappy persons who are periodically deprived of the use of their rational faculties, and restored to the enjoyment of them, are even in the language of our legislation styled lunatics, from the Latin term luna signifying the moon. By some curious investigations inserted a few years ago in the Transactions of the Royal Society of London, the influences of the moon on our atmosphere are shewn to be very unimportant indeed ; and the notion of her effects on the rational faculties of human beings seems to have arisen solely from a fancied correspondence in the periods of sanity and insanity with the period. ical motions and phases of the moon. One effect, however, produced by the moon on the earth, is a matter of more consequence, because its appearances are constant regular and susceptible of accurate computation. This effect is the change produced on the surface of the sea, rising and falling in an uniform manner, agreeably to the position of the moon with respect to the earth and to the sun. This rising and falling of the surface of the sea we call the tide, a phænomenon equally curious in itself and useful to mankind in promoting mutual intercourse and advantage, by means of navigation and commerce.

The true cause of the tide was pointed out by the celebrated German philosopher Kepler, about two hundred

[ocr errors][ocr errors]

years ago : but it was reserved for the immortal Newton to furnish a complete and satisfactory explanation of that very curious natural phænomenon. He shewed that a property existed in nature, hy which all bodies and their component particles mutually attract each other, in the direct proportion to the quantities of substance in them, but in the inverse proportion of the squares of their distances asunder. This property, called universal gravitation or attraction, produces this effect, that the parts of the sea directly turned towards the moon are drawn upwards from the usual level of their surface or in other words are drawn farther away from the centre of the earth, and consequently must be accumulated to a greater depth than usual on the side next to the moon. Again, the solid and central parts of the earth being more dense as well as nearer to the moon than the fluid parts of the sea, on the opposite side, which is turned away from the moon, these solid parts will be more powerfully attracted towards her than the more remote fluid parts ; hence these 'fluid parts will in a certain . sense be forsaken by the solid parts, and the sea will increase in depth, as much by the retiring of its bottom as if the surface had been raised above its ordinary level ; and consequently there will be two risings and two fallings of the tide in one rotation of the earth on its axis,' from one conjunction with the moon to another, in a little more than 24 8 hours. When the waters of the sea rise, they are said to flow, and when they fall they are said to ebb; the greatest elevation is called high water, and the greatest depression low water.

The sun greatly exceeding the moon in magnitude, the attraction of the particles forming his body ought to have a much more powerful effect on the earth than the mass of the moon; but the sun's distance from the earth is so great, in proportion to that of the moon, that the effects of his

[merged small][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small]

attraction are scarcely perceptible : such, however, as these effects are, they contribute, when the moon's attraction is exerted in the same direction, as at the new moon, to produce in general a higher tide than at the full moon, when these two bodies are on opposite sides of the earth, and the attraction of the sun acts in diminution of that of the moon. The high tides produced at the new and full moon are called spring tides; but at the first and third quarters of the moon, when she comes to the meridian of any place nearly 6 hours after the sun, their attractions counteract each other, and the difference between high and low water is the least possible; these are called neap-tides. As attraction requires some time to produce its effects, so we find that the highest point of the ride occurs always at a certain period after the moon has passed the meridian of the place of observation, and this even in the open sea; but in places on narrow seas, in mouths of rivers, and other confined situations, the motion of the water is so restricted that high-water does not recur until many hours after the moon's passage from the meridians of such places. The connection, therefore, between the time of high-water and the position of the moon, determined by the time of her being on the meridian, which in the northern hemisphere of the earth is called her southing, is a matter of observation only: from such observations tables have been formed, indicating the period when the tide is at its greatest height on the days of the new and the full moon, in different parts of the world; and of such tables the following is a specimen, exhibiting the time of high-water on such days, in some principal positions of the British isles. The first and second column contain the names of the places ; the third shews the hours and minutes when the tide is at its greatest height on the days of the new and the full moon.

[ocr errors]

PLACE

« הקודםהמשך »