LEVEL. 



difference between those two heights 

 will exhibit how much the former is above 

 the latter. 



By this simple mode the level of any 

 intended land, Sec. may be correctly taken, 

 observing to limit the sight as much as 

 possible : indeed, it is always best to con- 

 fine them to distances not exceeding 

 three hundred yards ; because the differ- 

 ence between the rectilinear and the 

 horizontal levels are then greatly dimin- 

 ished, and the whole survey will prove 

 far more correct. This will be easily 

 seen from the following sketch. (Plate 

 VIII. Miscellanies, fig. 2). Let A B re- 

 present a sight taken at 2100 yards, and 

 let A C be an equal distance measured on 

 the surface of the segment A D, but brok- 

 en off at every three hundred yards, i. e. 

 into seven portions : it must be obvious 

 that the line B C will give a greater length 

 than would result from the proportion al- 

 ready stated, the perpendicular falling so 

 much beyond D ; and that such differ- 

 ence would increase in its disproportions 

 according as the range of sight might be 

 enlarged. To prove this still more clear- 

 ly, let us state that the quadrant O P 

 (tig 3), of the earth's surface stands on a 

 radius P S of four thousand miles. Now 

 the first taken from the summit O of that 

 quadrant would be a parallel to that hori- 

 zontal radius, and a tangent to the arc at 

 its summit, as from O to X. It is evident 

 that if a sight of four thousand and one 

 miles could be taken in the direction O 

 X, a perpendicular falling from X would 

 not even touch the point P, from which it 

 would be a mile distant. It is true, that 

 our sights are not to be compared with 

 the foregoing extents; but it is equally 

 true, that we verge towards the error 

 ibove shown when we take too long 

 sights. 



Perhaps nothing can be more decep- 

 tive than the common mode of estimating 

 levels : more than once we have witness- 

 ed the opinions of smatterers in this, 

 branch of surveying, who have levelled 

 the instrument with great exactness, and 

 directed the telescope to a very distant 

 hill, on seeing the point of intersection 

 cut near its summit, have concluded the 

 spot so indicated to be on a level with that 

 where the instrument stood. This mis- 

 conception arises purely from a long rec- 

 tilinear sight, without considering that 

 the base of a remote hill is a plane, whose 

 surface stands at a very great angle from 

 that on which the level is placed; as 

 shown by the dotted lines representing 

 a hill R intercepted by the line of sight 



A very good kind of level is made on a 

 portable plan, by several mechanics in 

 this branch. It consists of a small tube of 

 glass let into a plate with which it is ex- 

 actly parallel. This is the surface, shew- 

 ing the bubble in the tube, as before de- 

 scribed: the under part of the box, 

 which may be about eight inches long, 

 two broad, and two deep, has a spring 

 and screw that cause the box to change 

 its direction from either above or below 

 the horizon, to an exact rectilinear level, 

 as indicated by the air left in the glass 

 tube. At the centre of the bottom of the 

 box is a brass stud, serving to fix into a 

 hole made in the top of a walking-stick, 

 Stc. This kind of level, in the hands of 

 a skilful surveyor, may be used to great 

 advantage, where very great precision is 

 not required ; but as the sight is taken 

 only from the upper edges of the box's 

 ends, it does not admit of that great nicety 

 which is indispensable in many opera- 

 tions, and where the smallest deviation 

 from a true level might occasion immense 

 expense and inconvenience. 



The level represented in Plate Level, 

 was made by the justly celebrated Jesse 

 Ramsden, F. R. S. and considered by him 

 as the most complete. It stands on three 

 legs, which fold up into the size and ap- 

 pearance of a moderately thick round 

 staff; three sliding rings, or leather bands, 

 suffice to keep them compact and firm. 

 The upper parts of the legs fit into a 

 brass plate, as in theodolites, on the sur- 

 face of which is a strong male screw, 

 serving to fasten the working part of the 

 instrument at pleasure, to its centre. The 

 female screw is cut withinside a projec- 

 tion b b of a brass plate a a -. this projec- 

 tion has a hole through its top, and con- 

 tains a brass ball (f, screwed into an- 

 other similar plate e e ; by which means 

 the two plates are connected together, 

 and the upper one can be turned about in 

 any direction, while the lower one re- 

 mains fixed. Four screws n passing 

 through the lower plate, being worked in 

 until they touch the bottom of the upper 

 plate, serve either to fix the latter firmly ; 

 or, by unthreading one or two, and work- 

 ing in the opposite ones, to change the 

 inclination of the upper plate, and to 

 bring it to an exact level. These are 

 called the parallel plates, though they 

 often stand at an angle, the one with the 

 other, when the direction of the upper 

 one requires changing as above shown. 



The ball d is perforated with a conical 

 hole, to receive an axis /, that is screwed 

 to the bottom of the compass box, /, on 

 which are two square brass arms FC, pro- 



