4?24? Mr. Nixon's Theory of the Telescopic Level. 



When in use, the telescope would rest within a trough formed 

 by the junction of two planes (equally inclined to opposite 

 points of the horizon, and secured to the surface of the upper 

 parallel-plate of a tripod. In order to diminish the weight, 

 without affecting the accuracy of the instrument, the artist 

 makes a section of the trough, perpendicular to the line of 

 junction of its sides, within a short distance of each end, and 

 removes the whole of the intermediate part. These sections, 

 or notches, (called Ys from their resemblance to that letter,) 

 will be exactly opposite and parallel to each other, and have 

 the same angular opening. 



A spirit-level is a closed glass tube, nearly filled with spirits 

 of wine, fixed to a straight bar, of which the under surface is 

 a plane. A line drawn on this under surface in the direction 

 of a plane passing through the axis of the tube, we shall call 

 the reversing line of the level. 



(For definitions of vertical, horizontal, and inclined lines and 

 planes, see " The Theory of the Spirit-level," Phil. Mag. and 

 Annals, N.S. vol. i. p. 256.) 



The reversing line of the level being placed, in opposite 

 directions, in contact with a horizontal line, the bubble, con- 

 sidered as a point, will come to rest, in both instances, at the 

 same point of the tube called the reversing point. A level 

 may be placed in any direction in contact with a horizontal 

 plane, without displacing the bubble (from its reversing point). 

 When a level can be placed in contact with a plane in any 

 two directions at right angles to each other, and reversed in 

 both without deviation of the bubble, that plane is horizontal. 

 When the reversing line of a level is placed in two opposite 

 directions in contact with an inclined line, and the place of 

 the bubble is marked in both instances on the tube, the di- 

 stance between the two marks, converted into an arc of a 

 circle, will be double the horizontal inclination of the line; 

 the reversing point lying between and equi-distant from the 

 two marks. In a revolution of the level in contact with an 

 inclined plane, the bubble would be twice at the (reversing or) 

 same point of the tube ; occurring when the reversing line of 

 the level was in the direction, and in the opposite direction of 

 one of the horizontal lines, which may be drawn (parallel to 

 each other) through any point of an inclined plane; and 

 twice, when at right angles to these directions, the bubble 

 would be at the same and maximum elongation from the sta- 

 tionary or reversing point, but on opposite sides of it; the 

 equivalent angle of the linear space passed over by the bubble 

 being the double of the horizontal inclination of the plane 

 and line. Parallel lines are either all inclined, or all parallel 

 to the horizon. 



When 



