Improved Agricultural Drainage Levd. 



167 



position. In the centre of this bar is a universal joint that will 

 allow the telescope to be turned in any direction, and at this joint 

 the level is fixed to the tripod stand, which is one of a very simple 

 and firm construction. 



The method of adjusting the Instrument, and reading the Staff. 

 ■ — First place the level on the stand and direct a person to hold a 

 graduated staff perpendicularly at a convenient distance, say 50 

 yards. Direct the telescope to the staff, draw out the eyepiece 

 a little by the edge a a, till the cross lines are seen clearly, then 

 draw out the telescope tube by the edge h h, till the divisions on 

 the staff are also clearly seen. 



Move your eye a little up and down while looking through the 

 telescope, and if the horizontal line seems to flit or move amongst 

 the divisions on the staff*, slide the telescope tube a little in and out 

 by the edge h h, till the horizontal line, while the eye is being moved 

 up and down, seems fixed upon one part or division of the staff: 

 if, by doing this, the divisions on the staff have become a little 

 indistinct, slide the eyepiece a a a. little in or out, till the divi- 

 sions again appear distinct. This latter operation is termed cor- 

 recting the telescope for parallax, is a very easy one, and is 

 very necessary to be carefully done where accuracy is required. 



Move the telescope on the universal joint till the staff appears 

 between the two vertical lines and parallel 

 to them ; at the same time put the telescope 

 as nearly into the horizontal position as 

 possible, then with the screw d bring the 

 spirit bubble exactly into the middle of the 

 opening. 



The instrument is now correctly adjusted, 

 and while in this position the reading of the 

 staff should be taken. 



The telescope inverts objects, but any 

 difficulty from this, to a person unac- 

 customed to such a telescope, will be 

 easily removed by considering the divisions 

 must be read in the direction in which 

 they are numbered, or apparently down- 

 wards. 



Thus if the horizontal line in the tele- 

 scope appears to cut the staff" at a, the 

 reading is 17 inches; if at b, it is 22l 

 inches, &,c. 



If the level move too stiffly or too easily on the universal joint, 

 it must be altered with the milled screw, ee. 



Levelling, as required for Agricultural Purposes. — Levelling is 

 the art of finding a line parallel to the horizon at one or more 



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■20 



■d 



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