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SUNDIALS IN TOURAINE AND ELSEWHERE
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BAGNOLD SUNCOMPASS

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This is a small instrument of great precision. It has the great advantage that the shadow plate itself is graduated into 360 degrees of bearing, so the shadow shows directly and continuously the actual true bearing in wich the vehicle is heading. Hence the heading can be read off at a glance and logged at any moment without re-setting the instrument.


PRINCIPLE:

The 360° shadow circle is movable. When set with the 0° mark towards the front of the vehicle,making an angle of 180° with the tail of the vehicle, the sun at noon will cast a shadow on the mark wich will give the true heading, whatever that may be. At other times of the day the shadow will give the true heading if the 0° mark makes the same angle with the tail of the vehicle as the sun's azimuth at that time. In other words, the 0° mark must must be made to move round with respect tothe lubber line (the for-and-aft axis of the vehicle) at the same rate as the sun's azimuth changes through the day with respect to the meridian.

An azimuth scale is therefore marked on the side of the shadow plate, and the lubber line is shown by a fixed indes set accurately towrds the tail of the vehicule. The shadow plate is rotated by a thumb screw so arranged that one complete turn changes the azimuth of the plate by exactly 2°.

The sun's azimuth, cast and west for every half hour 'and for every 15 and 10 minutes when the sun is very high) is given for each latitude by a separate card which contains all the necessary figures for the whole year.

The azimuth figure on the card for any given half or quarter hour period is computed for the middle of the period. So there must be a certain small error in the bearing towards the beginning and end of the period. But since these errors are in opposite sences they cancel out exactly over the whole period. The year is divided into calendar periods, and the figures are again computed for the mean date in each period. The period are so arranged that the error due to the change in the sun's declination between the ends and the middle of a period is never more than 2°. For very accurate work the correct figures for the sun's azimuth can be interpolated for any given date.

A separate card is provided for every three degrees of latitude between Lat 36° and Lat 15° N. By a suitable change of dates the cards can be used for the same latitudes in the southern hemisphere.

The change of azimuth per degree of latitude is given against the azimuth figures, so accurate corrections can be made for changes of latitude during a journey.

The only disadvantage of the Navigating Sun-Compass is that it requires separate cards which may get lost. Since however this compass is intended for the use of trained navigators, who should be intelligent and responsible men not given to losing things, this disadvantage is not serious.