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OCR Rendition - approximate278 Art of Travel. aim must be just twice as accurate, neither more nor less, as would suffice to point a rifle at the sun when it was sufficiently obscured by a cloud to bear being looked at : for the object of the aim is of the same apparent size, but a movement of a mirror causes the ray reflected from it to move through a double angle. The power of these sun-signals is extraordinarily great. The result of several experiments that I made in England showed that the smallest mirror visible under atmospheric conditions such that the signaller's station was discernible, but dim, subtended an angle of only one-tenth of a second of a degree. It is very important that the mirror should be of truly plane and parallel glass, such as instrument-makers . procure; the index glass of a full-sized sextant is very suitable for this purpose : there is a loss of power when there is any imperfection in the glass. A plane mirror only three inches across, reflects as much of the sun as a globe of 120 feet diameter ; it looks like a dazzling star at ten miles' distance. To direct the flash of the Mirror.-There are makeshift ways
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