CHAP. I.] HELIOSTAT 37 



HELIOSTAT. 



This instrument, which is simply a mirror mounted in Most 

 gimbols, so as to turn and reflect the sun in every direction, is Y^^^^^ie 



» T . Adjunct 



01 great use. In a survey where many assistants are at work for Marine 

 together, it saves an immense amount of time. Smaller beacons ^^'^^^y- 

 or marks can be erected, and the position of a theodoUte station 

 that has to be made on the side of a hill, or with dense foHage 

 behind it, is at once made apparent to another observer, who 

 has to take angles to it by the flash, which can be seen a long 

 distance by the naked eye. 



Some heliostats supplied are mirrors in gimbols, mounted Fitting. 

 either on stands or in portable cases, with a spike to drive 

 into the ground. 



Neither of these forms is satisfactory, as in many places 

 from which it is desired to use them they cannot be con- 

 veniently and firmly placed. Tripod legs of some description 

 on which to place the mirror are best, and a movable arm 

 working round the centre, and carrymg an adjustable ring 

 through which to direct the flash, will be found very handy. 

 If the surveyor has to trust to placing some separate object, 

 such as a stick or another tripod, a few feet from the mirror, 

 by which to direct his beam of light, he will soon find himself 

 in some position where there is no standing-ground for such 

 object, as when his theodolite is on the top of a sharp hill, or 

 on a steep coast-line under cliffs at the edge of the sea. 



A better instrument is the excellent and convenient Galton's Galton's 

 Sun Signal, now also supplied. This is fitted with a telescope, signal, 

 by looking through which and adjusting the mirror, a dim 

 image of the sun is seen covering the object required to flash 

 to. Nothing can be better adapted to the purposes of the 

 nautical surveyor's work than this (when he is once accus- 

 tomed to it, as at first it is a little awkward to manage), and 

 when obtainable they should always be used. Care must be 

 taken, however, that the instrument is in adjustment. This 

 can be ascertained as follows : Place a board, with a sheet of 

 white paper pinned on to it, about 50 yards off. Direct the 

 sun signal flash on to it, and looking through the telescope, 

 screen and unscreen rapidly with the hand the direct flash 

 from the mirror. If the circular image formed by the direct 



