CHAP. I.] TEN-FOOT POLE 41 



A table of distances, corresponding to the angle subtended 

 by the length of the pole used, should be in each assistant's 

 possession for reference on the spot.* 



Fig. 8 represents a 10-foot pole. 



It is important in using a 10-foot pole to have regard to the 

 direction from which the hght falls on it ; when the sun is 

 low and behind the pole, it is difficult to obtain satisfactory- 

 results. 



In order to get a longer base without the inconvenience of 

 an unwieldy pole, the wooden frames may be connected by 

 a wire cord instead of by a pole, which when stretched taut 

 gives a distance of exactly 30 feet between the vertical stripes 

 on the two frames. 



Held firmly by two men, with the cord between them kept 

 quite taut, one stands fast and the other walks slowly back- 

 wards and forwards, to enable the maximum subtended angle 

 to be measured either by sextant or by theodolite. 



RANGE-FINDERS. 



The Barr-Stroud Navigational Range-finder is the one best 

 adapted for surve3dng purposes, and is likely to prove a very 

 useful adjunct to the equipment of a surveying vessel. 



DRAWING-BOARDS. 



In a surveying vessel it is convenient to have a considerable 

 number of these, and of various sizes, so as to fit all scales. 

 The largest may be about 29 inches by 25 inches. The size of 

 which most will be wanted will be about 27 inches to 25 inches 

 by 20 inches to 22 inches. There should be some smaller ones, 

 22 inches by 16 inches. 



Lightness, combined with sufficient strength not to warp, 

 is the requisite, and seasoned wood is therefore necessary. 

 White pine, | inch thick, is as good as anything, though 

 the smaller boards may be made of thinner mahogany. 



Duck covers to fit the boards are necessary for field work, 

 when the work is plotted at the time, to carry them in, and 

 prevent rubbing and wetting from rain. 

 * Appendix, Table R. 



