EXPERIMENT STATION BULLETINS. 



481 



TABLE I. 



Obsen^e: 1st. The elevation of point 1 had already been established 

 (or assumed). It should be recorded after stake I and under elevation. 



2. The back-sight, while recorded in the table after point 2, was 

 actually taken at point 1. It is always taken at a point whose elevation 

 has been established. It is best to record the back-sight on the line of 

 the stake numbered nearest the stake at which back-sight is taken. 



3. Back-sight reading 4.95 ft. added to the elevation of point 1, 11.35 

 ft. equals 16.30 ft. as the height of the instrument above datum. 



4. A fore-sight reading substracted from the height of instalment gives 

 the elevation of the point at which that sight was taken. The eleva- 

 tions appearing in the table are thus found. 



If there are other points too high or too low or too far away, to 

 fall within the range of the level, the level nuist be moved and set at a 

 new place or station, such that one or more of these other points shall 

 fall within its range and such, too, that one of the points vshose eleva- 

 tions have already been found shall also lie within range of the level. 

 The height of the instrument at this new position is now determined, 

 the back-sight reading being taken at a point within the range and 

 whose elevation is already found. The work from this point proceeds as 

 above described. ' 



Cautions : 1. Always before recording a reading observe the bubble in 

 the spirit level to be sure that the telescope is level. 



2. If at any time the level should be disturbed, it should be properly 

 set and its height determined before taking other fore-sight readings, 



USING CHEAPER KINDS OF LEVELS. 



The cheaper kinds of drainage levels are of necessity more "crudely" 

 made and cannot therefore be so delicately adjusted as the better made 

 and more expensive instruments. 



In leveling with these cheaper levels, usually only one fore-sight read- 

 ing is taken with each setting-up of the instrument. One back-sight 

 reading is also taken with each setting-up of the instrument, because 

 the back-sight must be had to determine the height of the instrument. 

 In using the cheaper level, the precaution should always be observed to 

 set the instrument nearly equidistant from the point whose elevation is 

 known and the point whose elevation is to be determined. In practice, 

 in leveling for drains where the fall is large, it is possible, with care, to 

 take two, three or even four fore-sight readings with each setting-up of 

 the instrument. But here as above the level should be set very nearly 

 midway between the point whose elevation is known and the farthest 

 point whose elevation is to be determined with this setting of the instru- 

 ment. 



If but one fore-sight reading were taken with each setting of the in- 

 .61 



