Equation of Time.

A good sun-dial affords a ready means of regulating a clock, and, for this purpose, the following numbers must be subtracted from the time as given by the dial, and the remainders will be the corresponding times that should be shown by the clock.

TABLE

Of the Equation of Time for every fifth Day.

m. S.

Thursday, May 1st, from the time by the dial subtract 3

Tuesday,........... 6th,

Sunday,......... 11th,

Friday,

16th,

Wednesday, 21st,

Monday,......... 26th,

1

3

35

Moon's Passage over the Meridian.

The transits of the celestial bodies over the meridian of a place are among the easiest of astronomical observations, when the instrument is correctly fixed; and, as exercises for the young observer, the following times of the Moon's passing the meridian may be pointed out:~

May 2d, at 26 m. past 5 in the morning

3d, 12

669

7

8

9

10

11

PHENOMENA PLANETARUM,

Phases of Venus.

Venus still continues to decrease in the breadth of her illuminated disk, but to increase in her apparent brightness, in consequence of her approaching the earth.

May 1st, {Dluminated part = 10-17046 digits

Dark part

= 1.82954

Eclipses of Jupiter's Satellites.

The proximity of Jupiter at this time to the Sun, renders most of the eclipses of his satellites invisible; and there will only be one of the second satellite seen this month, and none of the first. This will take place at 27 m. 10 s. after 8 in the evening of the 6th.

These will neither be numerous nor much varied this month. In those that do happen, the Moon will be the chief agent. Saturn, however, will be in quadrature at 5 in the afternoon of the 4th, and Mercury will obtain his greatest elongation on the 28th. The Moon will be in conjunction with this planet at 24 m. past 4 in the afternoon of the 11th; with Venus, at 5m. after 2 in the morning of the 13th; with a in

Scorpio, at 6 in the morning of the 25th; and with Georgium Sidus, at 38 m. after 1 in the morning of the 28th.

DESCRIPTION AND USE OF ASTRONOMICAL

INSTRUMENTS.

[Continued from p. 101.]

THE TRANSIT INSTRUMENT.

These instruments were originally made so as only to be fixed in regular observatories; but many of a portable kind have now been constructed, which may be used in any place for ascertaining the rate of a clock or chronometer, and also for determining right ascension, when correctly placed in the meri

dian. One of the most convenient of this portable construction is represented by the above figure, and is one of the numerous inventions of that excellent artist, Mr. Troughton.

In this instrument, the telescope is 20 inches long, its aperture 14 inch, and the magnifying power from 20 to 35, according to the eye-piece that is employed, which is sufficient for observing the pole star in the daytime. A thick brass ring forms the base of the instrument, and has three strong and equidistant screws for feet. The two vertical frames, that support the axis, are also fixed to this base by screws, and kept perpendicular by bracing bars, which are attached by screws to the base and pright frames respectively. The base being circular, it is not only firm in its support, but preserves its figure in all degrees of temperature, which is an essential quality in such instruments. As the inner diameter of this ring and the length of the axis are each a foot, and the height of the supports about 13 inches, when the screws are taken out, the whole packs up into a small box, and is very portable. The graduated circle (the edge of which is seen on the left) is six inches in diameter, and admits of reading to minutes, by each of two opposite verniers, which is sufficient for finding the meridian altitude of any celestial body whose declination is known, and the latitude of the place of observation given; or for finding the latitude, to the nearest minute, from an observed altitude. It was not, however, the inventor's object to adapt it particularly for taking altitudes, and he therefore limited himself to such properties as were requisite for constituting a useful and portable transit instrument.

The level belonging to this instrument is detached, and is equal in length to the axis upon which it is to be placed when used, and has notches in its enpieces for this purpose. By this means the level is reversed without inconvenience; but it must be removed when the observations are made, to prevent its being displaced and broken by the elevation or depression of the telescope. There are also studs

of brass with conical holes in them for receiving the points of the screws, which serve as feet to the in strument: these are made fast in the stone or marble slab upon which the whole apparatus usually rests. Besides these, there are also darkened glasses, lantern, and other minor appendages, which are requisite for completing the use of the instrument under all circumstances. Several other portable instruments of this kind have also been made by other artists; among these, one with cast iron frames, by Mr. Jones, of Charing Cross, may be mentioned; which, having 30 and 42 inch telescopes, has greater power than the one above described, and is cheaper in proportion to its size.

The adjustment of the various parts of this instrument require some explanation: It is the business of the instrument-maker to adjust the level, but this may be easily tried by placing it on the axis of the telescope, and then, if the bubble rise to one end, that must be lowered either by turning the proper screw, if there be one, or by deepening the notch, till the bubble is brought to the middle; then, if the level be reversed, and the bubble still remain in the middle, the adjustment is complete, or the level is parallel to the axis of the telescope; if not, the same means must be repeated for bringing it to that position. It is to be remarked, however, that in the detached level, when the notches which rest upon the pivots of the telescope are properly made, they will seldom require a second rectification. The very same operation will also serve for bringing the axis of the telescope into its proper horizontal position; for when the level is placed upon the pivots, and the bubble brought to the middle by turning the proper screw, if it still remain in that position when the ends of the level are reversed, the axis will be horizontal. When the previous adjustment of the level can be depended upon, it will be sufficient to bring