Measuring Surface and Solidity. 247 



frequently, for the slightest elongation is detected in the bad 

 closing of the new with the old work. 



The twisting and hauling of the chain, so as to measure off- 

 sets, is one great cause of the alteration in its length. The 

 accuracy, and also the speed of the work, is increased, by mea- 

 suring the offsets by a tape line carried along with the chain. 

 Such an instrument should be 200 links long, and be contained 

 in a light box, eight inches diameter, capable of being fastened 

 by belts to a person's right side. The line should be fastened 

 to a cylinder of wood five inches diameter, and be received 

 into a groove formed of tin-plate inside the box, and turning 

 with the cylinder. By this means it is quickly wound up, and 

 its margins are prevented rubbing on the sides of the box. The 

 aperture through which the line enters the box should be in 

 hardened steel. If the tape, immediately after being drawn 

 through the linseed oil varnish, be stretched as tight as possible 

 between posts in the open air freely exposed to wind, and thus 

 kept till dry, it will retain its length after being divided, and 

 never require adjustment. When receiving the divisions, it 

 should be kept in tension by a weight of from ten to twelve 

 pounds. 



The measurements admit of being written in the field-book 

 in a uniform manner. The book is ruled in the usual way, 

 with three columns, the middle column for the lengths of the 

 surveying lines and the positions of the offsets, the two outer 

 columns for the offsets to the right or to the left. The sta- 

 tions are all designated by numbers ; and, beginning with the 

 first at the bottom of the page and writing upwards, every 

 subsequent measurement has its place in a continuous order. 

 It is thus impossible to neglect any of the dimensions in the 

 calculations or plan, for in them, the lines, with their bearings 

 and offsets, are taken in the same continuous order. 



In most surveys it is economical to measure all the lines and 

 offsets before the bearings ; and thus weeks, or even months, 

 may intervene between the two operations. For the stations, 

 marks are thus required, which can be easily identified, and 

 which also may remain in the ground for a considerable time. 

 For this purpose, pins of soft fir- wood, about three inches long, 



