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their comparison of the planet Mars with the star A in Aquarius. Mars being on the meridian of the Cape of Good Hope, Lacaille found that his zenith distance was 25° 2', and the northern limb of his disc 26".7 north of the star. At the same time, and under the same meridian, Wargentin observed Mars on the meridian at Stockholm, and found his zenith distance equal to 68° 14', and the same part of this disc south of the star 6′′.6. The angle formed at Mars by the two visual rays from the observers, was therefore 26".7+6".6=33".3. Now the natural sine of 68° 14′ is 0.9287, and that of 25° 2′ is 0.4231, and their sum is 1.3518. This is the denominator, according to the above rule, and 33".3 is the numerator. Hence the horizontal parallax of Mars, ac33":3 cording to these observations, is =24".64. 1.35 18 The numerator in this expression being so very small, it is not essential to have the sines of the zenith distances to the greatest degree of accuracy, and consequently the zenith distances themselves; and it is this which constitutes the advantage of this method.
The circumstance of the two observers being on the same meridian, appears at first greatly to limit the use of this method; but this may be avoided by observing the meridian altitude of the heavenly body for several successive days, from which its diurnal change in declination will be determined. Then if the observers are not under the same meridian, the change in the declination of the body during the time elapsed between its passage over the two meridians may be easily calculated, and the two observations referred to the same meridian by means of the result, and the parallax concluded as above. It was by a similar method that the parallax of the Moon was determined, by comparing the observations of Lacaille, at the Cape of Good Hope, with those of Lalande, at Berlin.
The parallax of the Sun, however, is so small, on
account of his distance, that none of the preceding methods will give it with sufficient accuracy, and it long exercised all the ingenuity of astronomers in ascertaining it. This, however, has now been found to be about 8.78; but we must omit any account of the method employed till we have an opportunity of explaining the elements from which it is deduced.
In the preceding observations, the earth has been considered as spherical, and the distance of the body of which the parallax was to be found as constant; but if either of these be subject to variation, the parallax will also be variable. And it is now generally admitted that the labours of astronomers have fully proved the equatorial diameter of the earth to be greater than the polar axis by about th part of the former; and therefore the horizontal parallax at the equator exceeds that at the poles in the same ratio.
The greatest parallax is that of the Moon, which, under the equator, is about 1o, or 3600". Therefore, by taking the 309th part of this quantity, we shall have 11′′.6 for the excess of the one above the other. For all intermediate latitudes the difference of the radii is less, and the parallax varies as the square of the sine of the latitude. But this difference, though very small, may have a considerable influence on various astronomical phenomena; upon the time of an occultation of a star by the Moon, for example, or even upon its possibility. It is, therefore, necessary to take it into the account in all delicate calculations.
Since parallax depresses the true places of the celestial bodies, it not only diminishes their altitudes, but alters both their horary angles and their polar distances. The changes which it produces in these elements constitute what astronomers denominate parallax of right ascension and parallax of declination. These may both be readily deduced from the parallax of altitude: but our limits will not permit us to enter into an explanation of the method at present.
The following easy and practical rule will give the parallax of the Moon for any required altitude, corresponding to the mean temperature and pressure of the atmosphere, when the horizontal parallax is known; and this may be obtained for every day at noon and midnight from the Nautical Almanack for the year.
Add the logarithmic sine of the horizontal parallax and the logarithmic cosine of the Moon's altitude together, omitting 10 in the index, and the sum will be the logarithmic sine of the parallax corresponding to that altitude.
The following example will make the rule still clearer. Required the Moon's parallax answering to 30° of altitude, the horizontal parallax at the time being 55'. Log. sine 55 8.2040703 Log.cosine 30o = 9.9375306 Parallax required 47′ 38′′ = 8.1416009
The Naturalist's Diary.
It is the season sweet, of budding leaves,
Of days advancing tow'rds their utmost length,
THIS month is usually considered as the most delightful of the whole year, and has long been the Muse's favourite theme; although much that is said of its beauties applies better to more southern climates, or, indeed, to our month of JUNE, which is, commonly, entitled to all the praises that the poets have lavished upon MAY. This month, however, is remarkable for the profusion of verdure which it exhibits: nature's carpet is fresh laid, and nothing can be more grateful than to press its velvet surface. The scenery of a May morning is, not unfrequently, as beautiful as possibly can be conceived; a serene sky,
The simple ayre, the gentle warbling winde,
a refreshing fragrance arising from the face of the earth, and the melody of the feathered tribes, all com-. bine to render it inexpressibly delightful, to exhilarate the spirits, and call forth a song of grateful adoration.
Behold the merry minstrels of the morn,
This delightful picture, however, is in our northern climate confined, but too often, to a few days of the month; or sometimes, indeed, not realized till June or July; this was particularly the case with the very backward spring of the past year; when May might, with great truth, have been addressed in the language of an anonymous poet:
Why com'st thou, gentle May, with driving rain
And chilly blasts, to check the opening year?
Why frown in sullen sadness, dark and drear?
Beam his glad radiance on the dewy flower;
Known the mild influence of one sunny hour.
With infant joy, the merry month of MAY;
In happiness and peace I passed the day.
Spring, however late may be its approach, is rendered doubly welcome by the anxiety with which we have expected her, and the clouds and storms that
have preceded this flower-crowned lady, like the misfortunes and disappointments of life, have only served to make its enjoyments more intensely felt. Light and shade constitute the harmony of the moral, as well as of the physical, world. We cordially hail this season with MR. WILSON, in his beautiful Hymn to Spring:'
Thou cam'st at last, and such a heavenly smile
The latest species of the summer birds of passage arrive about the beginning of this month. The goatsucker, or fern-owl (caprimulgus Europæus), makes its appearance only in the dusk of the evening, to search for prey, uttering a dull jarring noise. The spotted fly-catcher (muscicapa grisola), the most mute and familiar of all our summer birds, builds in a vine or sweet-briar, against the wall of a house, or on the end of a beam, and sometimes close to the post of a door. The sedge-bird (motacilla salicaria) is found in places where reeds and sedges grow, and builds its nest there, which is made of dried grass, tender fibres of plants, and lined with hair. It sings incessantly night and day, during the breeding time, and imitates, by turns, the notes of the sparrow, the skylark, and other birds, from which it is called the English mock-bird. The arduous time of incubation is now come, and birds are sedulously employed in hatching and rearing their young.
The insect tribes continue to add to their numbers; among these may be named several kinds of moths and butterflies (papilio atalanta, cardamines, ægeria, &c.) Spenser has the following beautiful stanzas on the butterfly: