of ascertaining the Heights of Trees. 



521 



plummet suspended from the apex, or centre, of the quadrant. 

 In using it, the operator must first measure a distance of 100 feet 

 from the tree, and then, taking the quadrant in his hand, apply it 

 to his eye, and look along the edge [a b) to the summit of the 

 tree. The plumb-line will cut the graduated edge at the exact 

 number of feet that the tree is high. The operator must then 

 add the height of his eye from the ground (say about 5 ft.); and 

 he must also notice whether the ground rises or falls towards the 

 tree, and make allowance accordingly. 



With respect to the principle on which the quadrant is gra- 

 duated, it will be easily understood by inspectingj^. 92. 



At the beginning of a scale of feet {a b) erect the perpendi- 

 cular line a c of a length equal to the distance the station is 

 intended to be from the object viewed. Upon this perpendicular 

 line describe a quadrant, taking the extremity c for the centre. 

 Then lines, drawn from each division of the scale of feet [a b) to 

 the centre (c), will form, where they cut the circumference of the 

 quadrant, the graduated scale required ; each part of which must 



