54 



The Dumpy Level. 



The dumpy level differs from the wye level in being attached to the level bar by 

 immovable upright pieces; in having the level tube firmly secured to the uprights 

 of the level bar, in being provided with an inverting eye-piece (unless ordered 

 otherwise), and in the absence of the tangent and slow-motion screws. In regard 

 to the level itself, and the optical power of its telescope, it is fully the equal of the 

 more elaborate wye level. 



Compactness is the object aimed at. with the dumpy level, and this must be 

 secured at the sacrifice of the parts of the wye level which may be considered more 

 in the light of conveniences than necessaries. 



Adjusting. 



A theoretically perfect dumpy level has the same points established that are 

 mentioned under the head of wye level ; but since its construction differs from the 

 wye level, the methods of adjustment are not so convenient, resembling closely the 

 adjustment of the transit telescope and its attached level. After adding the sun- 

 shade and setting up as nearly as level as may be, and setting the eye-piece so to be 

 rid of parallax, the two cross-lines should be set one at right angles to line the 

 telescope axis describes in its horizontal revolution, and the other cross-line parallel 

 to such a line. This is accomplished by loosening the four capstan-headed screws 

 near the eye-piece, and rotating the reticule until a point remains bi-sected when 

 the telescope is moved in azimuth. 



To adjust the level, bring the level over two of its foot screws, and bring the 

 bubble to the middle of its tube by means of the foot screws alone. Revolve the 

 instrument 180 in azimuth, and if the bubble remains in the middle it is adjusted, 

 if it does not, then correct half its deviation by the capstan-headed adjusting screw 

 at the eye end, and the remaining half by the two foot screws. Repeat the opera- 

 tion over the other two screws, until the instrument may be revolved in any position, 

 and the level bubble will remain in the middle of its tube. 



To adjust the telescope for collimation, any of the methods given for the 

 horizontal cross-line of the transit telescope (see page 48) will apply to the dumpy 

 level. The usual method is to use a sheet of water, or where that is not available, 

 two stakes which are driven with their surfaces in the same level plane. 



To make the adjustment with the stakes, set up the level half way between two 

 points Iving very nearly in a horizontal line, and say 300 feet apart. Point upon a 

 rod held at one of them, and bring the level to the middle of its tube. Drive a stake 

 At this point, and take the reading of the rod upon it. Point the telescope in the 

 opposite direction, again bring the level to the middle of its tube, and drive a second 

 stake at the second point selected until the rod held upon the second stake gives 

 the same reading as when held upon the first stake. The tops of these two stakes 

 now lie in the same level line. 



Take up the level and set it within a few feet of the first stake. Read the rod 

 upon the first stake, and then upon the second. Jf the two readings agree, and the 

 level is in the middle of its tube, the collimation is correct. If the two readings do 

 not agree, correct nearly the whole of the disagreement shown when the rod is 

 held on the distant point, by means of the upper and lower capstan-headed screws 

 near the eye end of the telescope, and repeat the operation until both rods read the 

 tiini-' with the level in the middle of its tube. 



The telescope and uprights are in a single casting, which is finished and fitt< <i 

 to the level bar, so that the line of collimation may be permanently parallel to it. 



The dumpy level will then be in adjustment, since the adjustment of its vertical 

 cross-line is of no importance. 



Adjustment of the Dumpy Level ' and. attached level of 

 Transit Telescope. 



Two-Peg Method. 



The following method is simple, direct, and geometrically accurate, requiring no 

 approximate measurement from apeg to the centre of lens, no trial setting <t tin- trie- 

 scope. * drive a peg just enough and not too far, and no auxiliaries except 

 level-rod and tape or chain. 



1 Contributed by Prof. R. Fletcher, Thayer School, Dartmouth College. 



