CHAP, ii.] AREOMETERS OR HYDROMETERS. 43 



at the highest point of the tube. If we place the tube on a metal 

 plate inclined towards the horizon, the bubble will rise to the highest 

 end of the tube : it will only remain exactly at the middle point if 

 the tube and the plate be in a perfectly horizontal plane ; the slightest 

 inclination in one direction or the other brings it to one or other of 

 the extremities of the tube ; to obviate this inconvenience, the tube 

 is slightly convex at its upper part, so that the movement of the 

 bubble is more rapid and decided towards this point. The horizon- 

 tali ty of the plane of the plate is perfect when the bubble, after a 

 few oscillations, remains so that its extremities occupy the same 

 divisions on either side of the centre of the convex top of the tube. 



To make a surface horizontal it is supported on three points 

 arranged at the angles of a triangle by levelling screws (Fig. 22) : 

 first a true level is obtained parallel to one of the bases of the 

 triangle, and by properly moving one of the two screws, the first 



Fro. 22. Horizontal of a plane obtained with a spirit-level. 



horizontal line is obtained. Then placing the level at right angles 

 to its first position, the third screw is used to obtain horizontally in 

 the new direction. The plane of the surface is then necessarily 

 horizontal, as two lines at right angles which are horizontal lie in it. 



When spirit is employed instead of water, much more accurate 

 observations are obtained ; hence spirit is used in preference in geo- 

 detic experiments and in levelling operations of importance, such 

 as the attachment of a level to an equatorial telescope to enable it 

 to be used as a transit instrument. All instruments of precision in 

 which certain portions must retain an exactly horizontal or vertical 

 direction during the observations are furnished with spirit-levels. 



