Oldalképek
PDF
ePub

fine crescent, with the convexity turned towards the Sun. She moves gradually westward with a retarded motion, and the crescent becomes more full. In about ten weeks she has moved 46° west of the Sun, and then she appears a semicircle, like our Moon in its first quarter, and her diameter is taken at 26". She now appears to become stationary, after which she moves eastward with a motion gradually accelerated, and overtakes the Sun about 9 months after having been seen on its disk. Some time after this she is seen in the evening, east of the Sun, round like a full Moon, but very small, on account of its great distance from the earth. She now moves eastward, and increases in apparent magnitude, but loses her roundness, till she gets about 46° east of the Sun, when she is again a semicircle. She now has a retrograde motion westward, increasing in diameter, but at length becomes a crescent, like a waning Moon; and at last, after a period of nearly 584 days, she comes again into conjunction with the Sun, with an apparent diameter of 59". The mean arc of retrogradation is about 16°, and its mean duration is 42 days. She does not move in the plane of the ecliptic, that is in the plane of the earth's orbit, but deviates from it several degrees.

The transits of Venus, or her passages across the disk of the Sun, observed from different parts of the earth, are very sensibly different in the times of their duration; a circumstance that arises from her parallax, which leads persons differently situated to refer it to different points of the solar disk, and of course it appears to them to describe different chords in its passage over it. In the transit which took place in 1769, the difference of its duration, as observed at Otaheite, and at Wardhuys in Lapland, amounted to 23 m. 10". The knowledge of this parallax enables us to ascertain that of the Sun, and consequently to discover its distance from the earth. Hence the transits of Venus have proved of great importance in astronomy. After succeeding each other in the in

[ocr errors]

terval of eight years, they do not occur again for more than a century, when they succeed each other again, during an interval of eight years, and thus they continue. The two last transits happened in 1761 and 1769: at both those periods astronomers were sent to different countries, where the observations were likely to be made under the most favourable circumstances, and it is from the result of their observations that the parallax of the Sun has been concluded to be equal, at his mean distance from the earth 16.6. Excepting such transits as these, Venus exhibits the same appearances regularly every eight years, her conjunctions, elongations, and times of rising and setting, being very nearly the same, on the same days as before.

The real diameter of Venus is nearly equal to that of the earth, being about 7680 miles, and her distance from the Sun is 68 millions of miles; her excentricity is 490,000 miles: the inclination of her -orbit to the ecliptic or orbit of the earth is 3° 23'; the points of their intersection or nodes are 15° of Gemini and Sagittarius; the place of her aphelion 9°: her axis is supposed to be inclined to the plane of her orbit about 75°, and her periodical course round the Sun 224 days 17 hours. The diurnal rotation round her axis is not well ascertained, being, according to Cassini, 23 hours; but according to the observations of Bianchini, it is 24 days 8 hours: this latter opinion is not generally admitted.

The phases of Venus to which we have already referred, were first discovered by Galileo, who thus accomplished the prediction of Copernicus; for when this illustrious astronomer revived the antient Pythagorean system, asserting that the earth and planets revolve about the Sun, it was objected that, in such case, the phases of Venus should resemble those of the Moon; to which Copernicus replied, that at some future time this resemblance would be found out. Galileo sent an account of the first discovery of these phases in a letter written from Florence, in the year

1

1611, to William de Medici, the Duke of Tuscany's Ambassador at Prague, desiring him to communicate it to Kepler. Having recited the observations he had made, he adds, we have the most certain, sensible decision and demonstration of two grand questions, which to this day have been doubtful, and disputed among the greatest masters of reason in the world. One is, that the planets in their own nature are opake bodies, attributing to Mercury what we have seen in Venus: and the other is, that Venus necessarily moves round the Sun, as also Mercury and the other planets; a thing fully believed, indeed, by Pythagoras, Copernicus, Kepler, and myself, but never yet proved, as now it is by ocular inspection upon Venus.'

In the Philosophical Transactions for the year 1792, are published the results of a course of observations on the planet Venus, begun in the year 1780, by Mr. Schroeter, of Lilienthal, Bremen. From these observations, the author infers that Venus has an atmosphere in some respects similar to that of our earth, and that it has mountains on its surface five or six times as high as those on the earth. Dr. Herschel, likewise, between the years 1777 and 1793, has made a great number of observations on this planet, accounts of which are given in the Philosophical Transactions for 1793. He says that the planet revolves about its axis, but the time of its revolution is uncertain: that the planet's atmosphere is considerable: that the planet has probably hills and inequalities on its surface, but he has not been able to see much of them, owing probably to the great density of its atmosphere : with respect to the mountains of Venus, no eye, he says, which is not considerably better than his, or assisted by better instruments, will ever get a sight of them. He makes the apparent diameter of Venus, at the mean distance of the earth, to be 18".79, which makes Venus larger than the earth, instead of being less, as was generally supposed.

E

The Naturalist's Diary.

Advancing Spring profusely spreads abroad
Flowers of all hues, with sweetest fragrance stored;
Where'er she treads, love gladdens every plain,
Delight, on tiptoe, bears her lucid train;
Sweet Hope, with conscious brow, before her flies,
Anticipating wealth from summer skies.

IP there has been a medium proportion of easterly winds in the previous part of the winter, the month of April may be expected to be mild, with gentle showers; thus affording to vegetables an abundant supply of water, which is so indispensably necessary to their existence. The many thousand tribes of vegetables are not only all formed from a few simple substances, but enjoy the same sun, vegetate in the same medium, and are supplied with the same nutriment. It is, indeed, wonderful that all orders of vegetables are produced from four or five natural substances, viz. caloric, light, water, air, and carbon. How admirable, then, must the formation of those organs be, which, by their peculiar actions, shall produce such various modifications of these substances, so as to form the different colours, tints, odours, tastes, &c. of the vegetable kingdom! How surprising must be the progress of vegetation! How rich the economy of nature!

The arrival of the swallow, early in this month, announces the approach of summer, and now all nature asumes a more cheerful aspèct. The swallow tribe is of all others the most inoffensive, harmless, entertaining and social; all, except one species, attach themselves to our houses, amuse us with their migrations, songs, and marvellous agility, and clear the air of gnats and other troublesome insects, which would, otherwise, much annoy and incommode us. Whoever contemplates the myriads of insects that

sport in the sun-beams in a summer's evening in this country, will soon be convinced to what a degree our atmosphere would be choked with them, were it not for the friendly interposition of the swallow tribe! Swallows are found in every country of the known world, but seldom remain the whole year in the same climate:

Long, little wand'rer, be thy stay
Within our sea-girt Isle !

And Summer yield her softest sweets
To pay thy pleasing toil!

And many a fresh returning year

Again survey thy swift career!

And thy early note again

Haply please the rural swain,

While twitt'ring o'er the straw-built shed'
Thou 'wak'st him from his lonely bed!'

DR. SHAW.

There are four species of the hirundines that visit England; they arrive in the following order:

I. The chimney swallow (hirundo rustica) builds its nest generally in chimnies, in the inside, within a few feet of the top, or under the eaves of houses: it is curiously constructed, of a cylindrical shape, plastered with mud, mixed with straw and hair, and lined with feathers; it is attached to the sides or corners of the chimney, and is sometimes a foot in height, open at the top. Swallows soon become familiar after they have been caught'.

2. The house martin (hirundo urbica), known by its white breast and black back, glossed with blue, visits us in great numbers. It builds under the eaves of houses, or close by the sides of the windows; and constructs its nest of mud and straw, lining it with feathers.

:

* One that had been taken and slightly wounded in the wing, so as to prevent its flying away, sat for its portrait to Mr. Bewick it remained on the bench while the wood-cut was engraved, and, from its having been fed by the hand with flies, while sitting, watched every motion; and at every look of the eye, when pointedly directed towards it, rau close up to the graver, in expectation of a fresh supply of food.-(British Birds, vol. 1, p. 264, 5.)

« ElőzőTovább »