38 HYDROGRAPHICAL SURVEYING [chap. i. 



flash on the sheet is not coincident with the image of the sun 

 as seen through the telescope, take off the cap at the end of 

 the tube and adjust with the screw that will be found under- 

 neath. 



A very workable arrangement can be fitted on board any 

 ship as follows : — 

 instni- A blacksmith will soon make a frame which will convert an 



easily im- ordinary looking-glass into a perfect instrument for surveying 

 provised work, as it must be remembered that we do not propose to 

 use it for talking, and therefore do not require the extreme 

 accuracy in directing the beam necessary in the military helio- 

 graph. 



The sketch annexed shows a looking-glass fitted in this 

 manner by a ship's blacksmith. The standard can be made 

 of any height as convenient ; about 2| feet is a good length. 

 In soft ground the end of the legs can be pressed into the 

 earth, and on rocky ground stones placed against the legs will 

 hold the instrument steady. The arm, m, of light iron, is 

 carried separately, and slips over the shaft of the standard, 

 clamping where required with a screw. 



Into a circular socket in head of standard shaft, the leg of 

 the frame holding the mirror is shipped ; this is also to be 

 tightened by a retaining screw. 



The mirror, which can be of any size from 2 to 6 inches 

 or more in diameter, revolves on its retaining screws, as an 

 ordinary toilet-table glass, and can be held in any position by 

 tightening these screws. 



The ring, of flat wood, is made as light as possible, so as to 

 exert less strain in wind. Across it are nailed crossed strips 

 of copper, with a white cardboard disc, about 1 inch in diameter, 

 fastened to their centre. 



The rod that carries this ring slips up and down in a hole at 

 the end of the arm, and is clamped by a retaining screw. 



In the centre of the back of the mirror, a hole of about | inch 

 diameter is scraped in the tinfoil, being careful to leave a sharp 

 edge. A similar hole is cut out of the wooden back of the glass 

 frame. This we shall call the " blind spot." 

 Using the To direct the flash to an object, bring the mirror vertical, 

 Heliostat. ^nd looking through the hole in the centre, revolve the arm 

 until in the direction of the object nearly, clamp it, and adjust 



