object, particularly astronomical objects, there is a small tube or telescope, called the finder, fixed near the eye-end of the large telescope. At the focus of the object glass of this finder, there are two wires, which intersect each other in the axis of the tube, and as the magnifying power is only about six times, the real field of view is very large; therefore any object will be readily found within it, which being brought to the intersection of the wires, it will then be within the field of the telescope.

In viewing astronomical objects, (and particularly when the greatest magnifying powers are applied) it is very necessary to render the telescope as steady as possible; for that purpose there are two sets of brass sliding rods, ii, as represented in the figure. These rods connect the eye-end of the telescope with two of the legs of the stand, by which any vibrations of the tube that might be occasioned by the motion of the air or otherwise, will be prevented, and the telescope rendered sufficiently steady for using the greatest powers. These sliding rods move within one another with so much ease, as to admit of the rack-work being used in the same manner as if they were not applied.

Of the Mariner's Compass.

THE Mariner's Compass is an instrument by which a ship is directed to any intended port. It is also of the greatest use in determining the bearing of one object from another.

The following figure is a representation of the card of a steering compass, with the names of eight of the points marked; the other divisions indicate half points.

The circumference of the card of the compass is divided into thirty-two equal parts, called points; the interval between two adjacent points is, therefore, equal to 11° 15', and each point is subdivided into quarters. In azimuth compasses, the circumference of the card, besides the usual division into points and quarters, is also divided into 360 degrees; and in some other compasses the quarter of the circumference is divided into 96 equal parts, and hence each division is 56 minutes. Four of the points of the compass, namely, N. E. S.W. are called cardinal points, being the principal points, and because the names of the others are derived from them.

To the under side of the card, and in the direction of its north and south line, is attached a magnetic bar of hardened steel, of a rectangular form, called the needle, by which means the north end of the card is directed towards the northern part of the horizon; and hence the other points of the compass are directed towards the correspondent points of the horizon.

The card and needle are suspended on an upright pin, called the supporter, which is fixed to the bottom of a brass or wooden circular box and the whole is covered with a plate of glass to prevent the wind from disturbing the card. The box has two pins, one on each side diametrically opposite. These pins are let into a brass ring, which is movable, in a square wooden box, on two pins at right angles to the former; by this contrivance, called jimbals, the card remains nearly horizontal, although the ship should be considerably agitated. Although the card be made to retain an horizontal position at the place where the compass was constructed, yet, when it is carried to any other place where the dip or inclination of the needle is considerably different, it will no longer remain horizontal. In order, therefore, to remedy this, one or two pieces of brass wire, with sliding weights, are placed below the card; and hence, by moving. one or both of these weights, the equilibrium of the card will be restored.

Upon the inside of the circular box a black line is drawn vertically, which line is usually called lubber's point. The compass should be so placed in the binnacle, that the line, joining the centre of the card and lubber's point, may be parallel to the ship's keel.

Before a compass is used it should be accurately examined. For this purpose it will be necessary to observe if the magnetic axis of the needle corresponds exactly with the north and south line of the card. This may be easily examined if the needle is so constructed as to be capable of reversion. Thus, let the needle be taken from the card, and suspended on a pin, and carefully observe the direction of its axis; reverse the needle, and again observe its direction; then half the difference of these will be the magnetic axis of the needle; and this line ought to coincide with the meridian of the card. Other methods might be proposed to determine the angular distance between the meridian of the card and the magnetic axis of the needle; the easiest of which is by comparing it with a meridian line, truly drawn on a plane; for the difference between the direction of this meri dian as shewn by the compass and the known variation is the error.

Description and Use of Hadley's Quadrant.

ONE of the most useful and convenient instruments for measuring the altitude of a celestial object, is Hadley's Quadrant, which is represented by the following figure.

The form of the instrument is an octagonal sector of a circle, and contains 45°; but, because of the double reflexion, the limb is divided into 90°.

A B, and AC, are the two radii, and BC the circle or limb, which, together with the braces, PQ, form the octant, or frame of

the quadrant; A is the index glass, H the fore horizon glass, and G the back horizon glass, and EF the corresponding sight vanes; I the coloured glass, the stem of which is put into the hole K, when the back observation is used; and L is a pencil for writing down the observation.

The altitude of any object is determined by the position of the index on the limb, when, by reflection, that object appears in contact with the horizon.

If the object whose altitude is to be observed be the sun, and if so bright that its image may be seen in the transparent part of the fore horizon glass, the eye is then to be applied to the upper hole in the sight vane; otherwise, to the lower hole: and, in this case, the quadrant is to be held so that the sun may be bisected by the line joining the silvered and transparent parts of the glass. The moon is to be kept as nearly as possible in the same position, and the image of a star is to be observed in the silvered part of the glass, adjacent to the line of separation of the two parts.

There are two different methods of taking observations with the quadrant. In the first of these the face of the observer is directed towards that part of the horizon immediately under the sun; and is therefore called the fore observation. In the other method, the observer's face is directed to the opposite part of the horizon, and consequently his back is towards the part under the sun, and is hence called the back observation. This last method of observation is to be used only when the horizon under the sun is obscured, or rendered indistinct by fog, or any other impediment.

This quadrant was first proposed by Newton, but improved, or perhaps re-invented, by Hadley. Its operation depends on the effect of two mirrors which bring both the objects of which the angular distance is to be measured at once into the field of view; and the inclination of the speculums by which this is performed serves to determine the angle. The ray proceeding from one of the objects is made to coincide, after two reflections, with the ray coming immediately from the other, and since the inclination of the reflecting surfaces is then half the angular distance of the objects, this inclination is read off on a scale in which every actual degree represents two degrees of angular distance, and is marked accordingly. There is also a second fixed speculum placed at right angles to the moveable one, when in its remotest situation, which then produces a deviation of two right angles in the apparent place of one of the objects, and which enables us, by moving the index, to measure any angle between 80° and 90°.

This operation is called the back observation; it is however seldom employed, on account of the difficulty of adjusting the speculum for it with accuracy. The reflecting instrument originally invented by Hooke was arranged in a manner somewhat different.

OF THE CALENDAR.

ALTHOUGH we have completed the systematic part of this work, we deem it necessary to add a short account of the Calendar, on account of its great importance in regulating time, and preserving the seasons and particular days to the same time of the year. And to render the work as complete as possible, we shall also add the method of performing the most interesting and important calculations in Astronomy; which we hope to do in a more simple and intelligible manner than what is to be found in works which profess to treat of this subject more fully than we intend to do.

Having already treated fully of the change of seasons and the regulation of time, we shall only observe here, that the tropical year exceeds the civil year, five hours, forty-eight minutes, forty-nine seconds. Now if this difference were not attended to, the seasons would soon happen in a different time of the year from what they do at present. This circumstance was known long before the real length of the year was ascertained; and to prevent it from taking place, the Romans inserted intercalary days; but without much regularity, till the time of Julius Cæsar, who observed that the year was almost hours longer than 365 days: he therefore added a day every fourth year, which made that year 366 days. This intercalary day was counted the 24th of February, and was called by the Romans, sexto calendas Martias, or the sixth of the calends of March; there was, therefore, in that year two sixths of the calends of March, whence it was called Bissextile.

To find Bissextile or Leap Year.

Rule.-Divide the given year by 4; if nothing remain, that year is leap year; but if 1, 2, or 3, remain, it is as many years after leap year.

Example.-Let it be required to find if the year 1825 be leap year, or the 1st, 2d, or 3d after it?

18254 456 with a remainder of 1; therefore it is the first after leap year.

Note. Even centuries not divisible by 4 are not reckoned leap years, such as 1800, 1900, 2100, &c.; but 2000, 2400, &c. are reckoned leap years.

OF THE DOMINICAL LETTER.

It has long been customary to distinguish each day throughout the year by one of the seven first letters of the alphabet; viz. A, B, C, D, E, F, G. The first, A, is affixed to the first day of January; the next, B, to the second; C to the third, and so on to the seventh, G; then A to the eighth, B to the ninth, &c. to the end of the year. By this means we know, that if any letter be prefixed to any day of

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