CLOUDS AND EIVERS, ICE AND GLACIEES. 63 



during its prosecution. Look then at this theodolite ; 

 it consists mainly of a telescope and a graduated circle, 

 the telescope capable of motion up and down, and the 

 circle, carrying the telescope along with it, capable of 

 motion right and left. When desired to make the 

 motion exceedingly fine and minute, suitable screws, 

 called tangent screws, are employed. The instrument 

 is supported by three legs, movable, but firm when 

 properly planted. 



156. Two spirit-levels are fixed at right angles to 

 each other on the circle just referred to. Practice 

 enables one to take hold of the legs of the instrument, 

 and so to fix them that the circle shall be nearly 

 horizontal. By means of four levelling screws we 

 render it accurately horizontal. Exactly under the 

 centre of the instrument is a small hook from which a 

 plummet is suspended ; the point of the bob just 

 touches a rock on which we make a mark ; or if the 

 earth be soft underneath, we drive a stake into it 

 exactly under the plummet. By re-suspending the 

 plummet at any future time we can find to a hairbreadth 

 the position occupied by the instrument to-day. 



157. Look through the telescope; you see it crossed 

 by two fibres of the finest spider's thread. In actual 

 work we first direct the telescope across the glacier, 

 until the intersection of the two fibres accurately 

 covers some well-defined point of rock or tree at the other 

 side of the valley. This, our fixed standard, we sketch 



