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vents these angles being taken nearer than 10°, which is 20o, on each side of the Meridian, and as the breadth of the observer's head interposes an equal hindrance to observation at the south end of the instrument, there are about 40° or 35°, at either end of the Pole, not to be reached by the Magnetic Quintant. If an object lie due magnetic North, such an object can be well noted; but in the event of the magnetic bearing being required of any object which falls within those limits, the observer must then take the magnetic bearing of any convenient object right or left of the Meridian, and, having noted that, then take the angle between that object and the object whose bearing is desired, and so arrive at the magnetic bearing of the object by a double operation.

2. WAYWISER.

THE Waywiser is an instrument whereby a man may steer himself where there is no path, or underground, in a mine; or find his way if he have lost it. The chief element in this use is of course the Magnetic Needle. The reverse side of the instrument is now employed. The Needle is uncovered. The legs are opened to an angle equal to the magnetic bearing of your destination from the Meridian. Holding the instrument now so that the Compass-leg lies north and south, you follow the direction indicated by the other leg of the instrument.

IX. AZIMUTH COMPASS.

An Azimuth distance is the observed position of the vertical plane through the Zenith and the Sun with respect to the meridian of the place in which the observation is made. At twelve o'clock, the equation of time, the Sun will be on the

meridian of the place; and if the Sun's bearing in the Heavens is observed at any other moment, the angular distance of the vertical circle in which the Sun is seen at that moment from the true meridian is called the Sun's azimuth.

To take a magnetic azimuth is to observe the direction in which the shadow cast by the Sun, either by a Line or a Lens, bears by Magnetic Compass at any hour of the day, when the Sun is visible.

To effect such an observation by the Polymeter:

1. The Compass-box must be turned over and the sight raised, and the instrument laid in the Sun in some convenient place, perfectly horizontal, as levelled by the Spirit-Level.

2. The Compass-leg being placed magnetic North and South, the Mirror-leg must be set at 90° the magnetic variation.

3. It will be found that when the Sun shines strongly a shadow will be very visibly thrown on the disc from the line which is engraved upon the Mirror. When the Sun is not so high as to make the operation impossible in this position of the instrument, this shadow will extend to and beyond the circumference of the disc; and the shadow of the scratch will fall upon some one of the degrees, and will at once give the

azimuth.

When this observation is made before mid-day, the shadow will be cast on the left-hand Quadrant of the disc; when after meridian, on the right-hand Quadrant.

This is a somewhat delicate operation and requires a good sun; but it is easily accomplished with a little practice.

When the Sun is so high in the Heavens that a shadow fails to be cast from the mirror on the disc, the instrument must be turned right about and the Compass-leg aligned to the true meridian. The operation in this case is performed by

adjusting the mirror's leg, and by sloping the mirror until the scratch on its polished back shall throw its shadow upon the disc at 0°. The eastern azimuth will be read off on the right hand by the half-arrow at the outer end of the Sector's leg. The afternoon or western azimuth will be taken by the reading of the corresponding numeration on the 3a quadrant of the disc.

X. MID-DAY DIAL.

To find true solar noon.

The instrument is to be laid perfectly level, with the Mirror-leg accurately East and West. The Mirror is to be set upright, with its face turned towards the Sun. The light of the Sun will be thrown by the Mirror on the inner rim of the annular disc. In this light will be seen a dark line, the shadow of the scratch on the Mirror's face. As the Sun comes towards the Meridian this dark line will be seen to approach to the zero mark on the rim.

is perfect it will be noon by the Sun.

When the coincidence

It is hardly necessary to say that this will not be true mean, or clock-time, but solar. To turn this into true clock-time recourse must be had to the table of equation of time which will be found in all almanacks. In order, however, that the observer may be independent of the book, there is engraved on the diagonal scale of inches a system of four curved lines, which are marked thus:

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These lines are to be used in correspondence with the Scale of

Equal Parts which occupies the edge of the instrument. Each of these Equal Parts now stands for three days. The intervals between the parallel lines of the diagonal scale stand for two minutes. Thus then, for every day (represented by the equal parts) the distance from the edge of the scale to the concave side of the proper curve will give the equation of time in minutes and fractions of a minute. The vertical stroke between the pairs of letters (which are the initials of the months) represent midnight on the last days of the months. For example, for the 12th of January. See on what curve the letters D. J. and J. | F. appear. The interval between the two vertical strokes is the month of January. From the graduation in the Scale of Equal Parts opposite to D. J. count four divisions to represent the twelve days, i. e. to the 12th of January; and note the point in the curve opposite the graduation so found. The length of "the Ordinate," that is to say, the distance of this point in the curve from the foot-line of the scale, counting each space it crosses for two minutes, will shew the equation of time for the day, being a trifle over four of these spaces, to be 8′ 37′′. Next, the observer will perceive a + mark at one end of the scale. This mark indicates that the equation of time found by the two curves distinguished by this mark are additive, viz. when the instrument gives mid-day by solar time, the clock must show 8 minutes 37 seconds past 12. That is, 8' 37" are added to the solar time to obtain true mean time. This then is the process for the two curves marked +, i.e. the first and third in page 26, which indicate additive Equation of Time. The other curves, the second and fourth, must have their ordinates measured in like manner from the other edge of the diagonal scale, by the same scale of equal parts; but the time, viz. minutes and seconds, indicated by the ordinate, will have to be subtracted from solar time,

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that is to say, when the Sun shews twelve o'clock, for example, on the 9th of November, the equation of time giving 16′ 2′′, the clock should shew 16′ 2′′ before 12.

By these processes we may obtain 12 o'clock with great accuracy, enabling the camps to have uniform and true time. To make the instrument available generally as a sun-dial we proceed as follows:

Set the instrument on the edge of the Mirror-leg carefully adjusted, level, and true North and South with the circular end turned from the Sun. Open the instrument to show the angle of the latitude of the place. Raise the sight-vane, carefully making it stand at right angles to the plane of the instrument. The Compass-box must be so turned that the sightvane shall be on the side of the instrument which is in the shadow. From a quarter before 8 o'clock till 12 the time will be read on the one side of the instrument; at 12 o'clock both sides of the instrument are equally in half-shadow. After 12 the Compass box will be turned round, and the sightvane will be ready to receive the shadows of the afternoon. In the centre of the sight-vane will be seen the hour-scale, marked 8, 4. 9, 3. 10, 2. 11, 1. with intermediate graduations for the parts of the hour. The equation of time is to be used as before. The Dial is good in any latitude in both hemispheres, if properly set. Before 7.45 A.M. and after 4.15 P.M. the sight-vane is not long enough to contain the whole shadow. The time may still be ascertained by the help of the square, thus: hold the square at the end of the sight vane perpendicular to it, that is, parallel to the leg of the Sector. Note, by the graduations on the edge of the square, the height to which the shadow of the top of the Sector-leg reaches upon it above the face of the sight-vane. Find the angle of which the tangent at two inches radius is equal to one inch minus the

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