688 



] ounial of Agriculture. 



[lo Nov., 1910. 



the same horizontal direction until it comes over the first pair of screws 

 but pointing in the reverse direction, again adjust the bubble. Repeat 

 over the second pair of screw^s, and if the instrument is in adjustment, the 

 bubble will now remain in the centre in whatever direction the telescope 

 may be pointing. Should this not be so, the instrument may be adjusted 

 as described later, but being a little out of centre will not vitiate results 

 so long as the bubble is brought to the centre when making the first and 

 final observations at any one station, and when taking readings on any 

 fixed points. Readings on ground surfaces do not, as a rule, matter to 

 half an inch. If running a line of levels as for a channel or drain, the 

 error of the instrument wdll l.>e corrected by setting it midway between 

 each backsight and foresight. 



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15. DUMPY LEVEL. 



The foregoing remarks apply equally to a three -screw instrument, only 

 when levelling the telescope, bring it first over a single screw, then over 

 the remaining two and repeat. It will also make it more convenient 

 for correcting the bubble when levelling along lines, to set a four-screw 

 instrument up with two of its screws approximately pointing in the direc- 

 tion of the survey ; and with a three-screw instrument, one of the screws 

 and the axis. No matter how carefully an instrument may be adjusted, 

 slight corrections of the bubble by means of the levelling screws will 

 constantly be required. 



The diaphragm of the telescope is furnished with one horizontal and 

 two vertical W'ires or cross-hairs as they are commonly called. In some 

 instruments these are made of fine threads taken from a spider in the act 

 of spinning, stretched across a metal ring and fastened with shellac. 

 In others, the diaphragm consists of a glass disc with similar lines marked 

 thereon and set in a metal ring, and are much to be preferred, especially 

 for amateurs, on account of their permanency. Spider webs are easily 



