44 American method of levelling for Sections. 



Avhen held on any points whatever, we obtain the heights of such 

 points above the datum. All such readings are called " fore- 

 sights," and registered in the third column marked F.S. In the 

 page of field-book which is here exhibited fore-sights are taken 

 on stations 1, 2, 3, and the readings are entered in the proper 

 column opposite the stations or chainages. We suppose that it 

 is impossible to take a reading of the staff if placed on the fourth 

 station, or that it is advantageous not to take such a reading, and 

 we direct the staff-holder to hold the staff carefully (and wave 

 it of course to get the least reading) on some firm ground, peg, 

 or stone in order that we may get the height of such point before 

 changing the position of the instrument. 



Having found such a point, say on a rock, we enter the reading 

 13 36 in the column of "fore-sights"; and then subtracting this 

 reading from 45 38 the height of the axis of instrument we have 

 3202 as the height of the point on rock above datum, which 

 height is entered in the fourth column. Then we take up the 

 instrument and set it in a favourable position from which to read 

 the staff held on the rock and to proceed with the work. Having 

 set the instrument carefully, we take a reading aay 2'34 of the 

 staff held on the rock, which reading must be entered in the 

 column of back-sights. Now it is obvious that this back reading 

 2'34 added to 3202, the height of the point on rock, will give the 

 height of the axis of the instrument in its new position. The result 

 of this addition is of course entered in the second column headed 

 "heights of axis of instrument"; and it is obvious that the 

 readings of the staff when held on any points whatever will, 

 when subtracted from 34'36 (the height of axis of instrument), 

 give the heights of such points above datum. We suppose we 

 can take readings on stations 4, 5, 6, 7, which readings are entered 

 as fore-sights opposite the stations. 



Now, before removing the instrument to a new position, we 

 find, as already indicated, a firm point and its height above the 

 datum, and we then proceed to carry on the work in like manner. 

 The new point entered in the field book is a new bench-mark on 

 a boulder 30 feet west of station 7, the reading on which is 1224, 

 and this taken from 3436 (the height of axis of instrument) gives 

 2212 as the height of such bench above datum. 



In carrying on the work, one important fact should be remem- 

 bered, viz: — That no reading on the staff is a "back-sight," 

 unless such reading be (he first taken after the instrument is set 

 in position. 



It may be proper to observe that the readings of "the staff on 

 the stations may or may not be reduced in the field during the 

 progress of the work, and that such reductions to the actual 

 heights above datum is not at all necessary in order to plot the 

 section. Indeed it will be found a great advantage to plot the 



