It is a good object by which to
adjust my hand heliostat, which I describe below. Two bits of paper and a
couple of sticks, arranged as in the drawing, serve pretty well to direct
a flash. Sight the distant object through the holes in the two bits of
paper, A and B, at the ends of the horizontal stick; and when you are
satisfied that the stick is properly adjusted and quite steady, take your
mirror and throw the shadow of A upon B, and further endeavour to throw
the white speck in the shadow of A, corresponding to its pin-hole in it,
through the centre of the hole in B. Every now and then lay the mirror
aside, and bend down to see that A B continues to be properly adjusted.
Hand Heliostat.--Some years ago, I took great pains to contrive a
convenient pocket instrument, by which a traveller should be able to
signal with the sun, and direct his flash with certainty, in whatever
direction he desired. I did so in the belief that a signalling power of
extraordinary intensity could thus be made use of; and, I am glad to say,
I succeeded in my attempt. I at last obtained a pretty pocket instrument,
the design of which I placed in the hands of Messrs. Troughton and Simms;
and upon the earlier models of which I read a paper before the British
Association in 1858. I called it a "hand heliostat." I always carry one
when I travel, for it is a continual source of amusement.
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