[EASUKEMENT OF THE DIAMETER. 



^ to 1^ inches are recorded as having a diameter of 1 inch, 

 those from 1^ to 2J inches as 2 inches, and so on. 



A good calHper must fulfil the following conditions : — 



(1.) It must be sufficiently light, so as not to fatigue the 

 labourer, and yet sufficiently strong to resist the wear 

 and tear, to which it is likely to be subjected. 



(2.) The two arms must be at right angles to the rule, or at 

 least parallel to each other, when pressed on to the tree. 



(3.) The movable arm must move with sufficient ease along 

 the rule. 



Callipers of iron would be too heavy and too cold in winter. 

 The former objection has been removed by making the in- 

 strument of aluminium. However, up-to-date callipers are 

 generally made of wood. As wood alters with the degree of 

 humidity, the movable arm is liable to jam at one time, or to 

 move too easily at others. To avoid this drawback, various 

 constructions have been adopted, resulting in a number 

 of callipers, of which the following two deserve to be specially 

 mentioned : — 



Giistav Heyer's Calliper. — The distinguishing feature of 

 this instrument is, that the rule 

 is given, in section, the shape 

 of a trapezium, and that it 

 is pressed up or down in the 

 movable arm by means of a 

 wedge, so as to counteract the 

 swelling or shrinking of the 

 wood. In Fig. 3, a represents 

 the cross- section of the rule, 

 h the wedge, and c the section of 

 the movable arm. The wedge 

 is fastened to a screw, which 

 can be moved by a key at (/. 

 On moving the wedge from 

 left to right, it presses the rule 

 upwards and thus tightens it ; ^.,^_ 3._iieyei's CaUiper. 



