40 



HYDROGRAPHICAL SURVEYING 



[chap. I. 



Power of 

 Penetra- 

 tion of 

 Flash. 



save time, after once adjusting the flash, to leave a bluejacket 

 to keep it on, while the surveyor is taking his angles ; and 

 although a man will soon pick up the knack, a larger mirror 

 will allow for eccentricities on his part, and also, on a dull 

 day, a faint flash will be detected from a large mirror, where 

 a small one would not carry any distance. 



On a bright day, a flash from a 3-inch by 2-inch mirror has 

 been seen fifty-five miles and more. 



In hazy weather, angles have been got when the place from 

 which the flash was sent was entirely invisible ; and thus whole 

 days have been saved by this simple contrivance. 



Only those who have spent hours, or even days, in straining 

 their eyes to see a distant mark can appreciate the value of a 

 heliostat. 



TEN-FOOT POLE. 



For coast-lining, a pole of measured length is often required, 

 to get distances by observing the angle subtended by it. 



A convenient form is as follows : — 



Two oblong wooden frames, about 18 inches by 2 feet, are 

 made as light as possible, and covered with canvas. These will 

 fit, by means of sockets at the back, on to the ends of a pole, 

 and copper pins passed through socket and pole will keep them 



Fig. 8. 

 Ten Foot Pole 



to feet. 



-^ 



at a certain fixed distance apart. Ten feet is a convenient 

 length for transport. 



The face of the canvas on the frames is painted white, with 

 a broad vertical black stripe in the centre, and the 10 feet will 

 be measured from centre to centre of the black stripes. 



In measuring with a sextant the angle subtended by such 

 a pole, the image of one stripe will be brought to cover the 

 other stripe. 



