Measuring Surface and Solidity. 245 



the old methods, but it is infallibly brought under the sur- 

 veyor's notice. 



In a survey of 10,000 or 12,000 acres, in order to prevent the 

 possibility of an accumulation of errors, it is sufficient to group 

 the stations by laying off the first bearing, by means of a power- 

 ful instrument, from a conspicuous point in the middle of the 

 ground, and communicating it to a number of other conspicu-r 

 ous points scattered over the ground, chosen rather from their 

 prominence and command of a good view, than from their fit- 

 ness to be the termination of survey ing- lines. From each of 

 these conspicuous points the bearings of the termination of the 

 surveying-lines within sight are again grouped, and finally the 

 bearings of the lines themselves are determined ; the object in 

 view being to derive every bearing from the first by the fewest 

 possible removes. If the first bearing has been the true north, 

 the position of the instrument may also be verified at any point 

 by again observing the direction of the pole-star while in the 

 meridian, or by taking equal altitudes of the sun. This bear- 

 ing, however, before comparison with the first, must be correct^ 

 ed for the inclination of the meridians to one another. The 

 correction in minutes is equal to the distance between the two 

 meridians in imperial links multiplied by the tangent of the 

 geometric latitude, and divided by 9206. By attending to these 

 precautions, the error caused by defective centering becomes 

 very small, and entirely unworthy of notice compared with the 

 unavoidable small errors in the length. 



In a survey of such a size as we have been contemplating, no 

 skeleton-work of triangulation is required. The result of the 

 whole, indeed, because of the corrections, may be supposed to 

 be derived from the continuous measurement of a base line 

 constantly verified. In the first extensive survey to which I 

 applied the system of co-ordinates, the ground was covered in 

 preparation by a series of large trigons, deduced at consider- 

 able expense from a base line measured with much more care 

 than the payment for common surveying work allows of being 

 used in ordinary lines. The positions of the stations were first 

 calculated by the usual trigonometrical formulae, and they were 

 afterwards reduced to the notation of co-ordinates for the sake 

 of convenient comparison with the subsequent work. The po- 



VOL. XXII. NO. XLIV. APRIL 1837. ft 



