CHAP, ni.] CHAINED BASES 69 



Check at every ten lengths by the number of pins. When 

 the tenth stroke is made, the foremost man should have no 

 pins left in his hand, and the other man should have nine, 

 the tenth having been just put in. 



The odd feet and inches in the last length are measured by 

 counting the Unks, which are each a foot long. 



In walking forward, take care that the hinder man does not 

 overwalk the former, or the chain will have a bight dragging 

 on the ground, links will catch in something and get bent, 

 and the error of the chain will be very different when retested, 

 to what it was before landing. 



The number of times a base must be measured depends on Repetition 

 circumstances. If for a harbour plan, only twice, if they agree 

 to a foot or two, will be sufficient. For a survey of greater 

 extent, three or four times \vill be more satisfactory, unless 

 the two first measurements agree very well. 



Perfectly level ground can seldom be foimd, and the sur- inequaii- 

 veyor must make an allowance for inequalities by his judg- 

 ment, which will be, of course, always subtracted from the 

 measured length. 



The chain must be tested for length, before and after 

 measuring the base, to ascertain the error. 



BASE BY DIFFERENCE OF LATITUDE. 



When two stations are available from twenty to thirty or 

 forty miles apart, visible from one another, and bearing not 

 more than two points from the meridian, having also a few 

 intermediate points visible from both, a very good base can 

 be got by latitudes, and careful true bearings. 



The base will then be diff. lat. x sec. Mercatorial bearing. 



Similarly, if the stations bear nearly east and west from 

 each other, the diff. long, may be obtained by chronometer or 

 rockets. The true diff. long, by observation is converted into 

 spherical diff. long, from which the departure is found. The 

 length of base will be Dep. x Cosec. Mercatorial bearing. 



By means of the intermediate points, triangles can be cal- 

 culated down to a workable length of side for fixing marks. 



Where no smooth ground for measuring a base can be found, 



