ASTRONOMICAL PHENOMENA. If these principles be understood, the reader will see that the answer to the following sum is correct : 4a-3bc ex + y - 5a2+be+ 2ex-2 -7a2+ 5bc ex + x Sum -8aε + 3bc Here, as was required, similar terms have been reduced to one term - ex of by the addition of the coefficients. It will be seen that the the first line, together with the -ex of the third, cancel the 2ex of the second line, and the result is nothing. As, a man who owes his creditors the very same amount as they owe him, gives nothing, and gets nothing: the debts are cancelled. SUBTRACTION, or the taking away of one quantity from another, is performed, by simply changing the sign of the quantity to be subtracted, and then proceeding as in addition. Thus, to subtract 56 from 96, Difference 96 56 9b-5b-4b. Again, to illustrate subtraction of minus quantities,— - 96 Difference 96 — (— 5b) — 96 + 5b = 14b. The reason of this will be obvious, on the consideration that, if the subtraction of one quantity from another produces a small result, while the subtraction of nothing produces a still larger result, the subtraction of less than nothing must lead to one even larger still. M. L. R. ASTRONOMICAL PHENOMENA. MARCH, 1855. By A. GRAHAM, Esq., Markree Observatory, Collooney. MERCURY will be in inferior conjunction with the Sun about half an hour before noon of the 6th. As he will then be near that part of his orbit most remote from the ecliptic, on the northern side, he will, relatively to the Earth, be several degrees northward of the Sun's disc, in passing from the eastern to the western side. He will be nearest to us on the 9th. His apparent motion among the fixed stars will be westward till the 18th, at 9h. in the afternoon, when he will be stationary. After this his motion is direct. On the 24th, a little after midnight, he will be in the plane of the ecliptic, going southward. At the close of the month he will have receded so far westward, as to rise an hour and a half before the Sun. VENUS is increasing in brilliancy, and apparently receding from the Sun. Her form is gibbous. She will be near the Moon on the evening of the 19th. MARS is close to the Sun, approaching him on the eastern side. JUPITER is also close, receding on the western side. The latter, towards the end of the month, will be visible to the naked eye before sunrise. 1 SATURN is slightly diminished in magnitude, in consequence of his increased distance. The ring retains nearly the same proportions, the outer axes being 40 and 18. The diameter of the sphere is only 16 less than the minor axis. The Spring Quarter commences on the 21st, at 4h. 8m. in the morning. RISING AND SETTING OF THE SUN, FOR THE PARALLELS OF THE BRITISH ISLANDS. SUN. SUN AND PLANETS AT GREENWICH. MERCURY. VENUS. MARS. JUPITER, SATURN. URANUS. H. T. & J. Roche, Printers, 25, Hoxton-square, London. THE YOUTH'S INSTRUCTER AND GUARDIAN. APRIL, 1855. THE MOSQUE OF OMAR. (With an Engraving.) THIS mosque takes its name from Omar, the first Mohammedan master of Jerusalem, the second Caliph, or Vicar, after Mohammed. His General, Abu-Obeidah, after taking Damascus, and other cities of Syria, sent part of his army to Jerusalem, by command of Omar, to lay siege to the city, in the year of our Lord 637. Ardently had Mohammed longed for "the Holy City," seat and burial-place of so many ancient Prophets; and now the conquest is attempted. The defenders of Jerusalem did valiantly, until, after ten days' hard fighting, Abu-Obeidah came with the remainder of his men, and sent a summons "to the chief commanders of the people of Elia," (as Jerusalem was then called,) "and to the inhabitants thereof," requiring them to testify that there is but one God, and that Mohammed is His Prophet. In that case, he would leave the city free. Or, if they refused this, he demanded tribute. If they neither consented to apostasy nor tribute, the enemy would destroy 'Caliph," or Khaleefah, is an Arabic word of exactly the same signification as Vicarius, or Vicar. The Caliphs were, in reality, the Vicars of Mohammed: the Popes, the Roman Caliphs, profanely call themselves Vicars of Christ. VOL. XIX. Second Series. H |