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34. A Simple Dendrometer — This instrument is constructed 

 essentially as follows : A tube, /, about two inches in diameter, and 

 nearly or quite a foot in length, is supported in a vertical position by 

 two nins. -hh near its; nni^f^r end t and these 



pms, pp, near its upper 

 ring, r, forming a simple gimbal which 



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weight to assume an 



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 sition. 



vertical po- 

 In the lower 

 3 end of the tube two 

 mirrorSj A and B^ 

 are placed side by 

 side, each occupying 

 one half of the sec- 

 tion of the diameter 

 of the tube. One of 

 these mirrors (^) is 

 inclined at an angle 

 of 45^ to the horizon 

 when the tube is in 

 a vertical 



position, 

 the other {B) 

 has an inclination of 



and 



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One side of 



the tube is properly 

 cut away (as shown 

 in the accompanying 

 figures) so that when 

 it is held vertically 

 the mirror A shall 



transmit to the eye 

 (at c) at the top of 

 the tube, the hor- 

 izontal ray a, and the 

 mirror j9, the ray h^ 

 which makes an an- 

 gle of 45^ with the 

 horizontal. 



The method of 

 using the instrument 

 is simple. By hold- 

 ing the supporting 

 ring, r, in the hand 

 in the proper posi- 

 tion, the centre of 



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the mirror A can easily be brought into the same horizontal line as 

 the base of the tree (represented by a) ; then, by moving towards or 

 away from the tree, its apex (represented by ^) may be brought into 

 view as it is reflected to the eye {c) from the mirror B. When both apex 

 and base of the tree are seen at the same instant, the distance from the 

 instrument to the tree along the horizontal line x-a is equal to the 



