There are various methods of ascertaining the heights of trees, all more or leas mlisfactory; but the aim pleat and moat efficient contrivance that has parallel line* are drawn at right angles to the edges. The square is attached by stout iron-shod pole about 4 1/2 ft. long - a convenient height for taking tree or other object intended to be measured, to the foot of the observer. The lines running perpendicular to the base line represent the altitude or height of the object either in feet, links, or yards, according to the stale by which the base line is measured. • The height of any given tree is indicated on the face of the dendrometer at the point where the plumb line (suspended from the point a) intersects the perpendicular line corresponding with the distance on the base line from the centre of the trunk of the tree to the observer. The figures along the top and bottom of the instrument show the number of divisions corresponding to the lines of altitude intersected by the plumb line.
This instrument is constructed on the principle which applies to all right-angled triangles. The side A B (Fig. 370) is termed the base-line, and corresponds with the horizontal line from the tree
Each line of altitude represented on the instrument corresponds with a nnit of the stale employed, whether this scale be in feet, links, or yards. The base line is marked only at every fifth unit, thus, 5,10, 15, 20, and so on. Whatever standard of measurement is fixed upon, whether it be in feet,, links, or yards, for the base line, it ia of course understood that the lines of altitude roust be fixed to the same scale. The divisions on the face of the instrument are 150, but if at any time it be desired to ascertain the height of an object above 150 ft., the divisions of the instruments most be termed yards, when, of course, a height of 450 ft. can be measured.
The mode of using the dendrometer is as follows: - Suppose the object to be measured be a tree. The operator must first place himself at a sufficient distance from the tree so that the extreme top of it can be distinctly seen. Note most then be made of the distance from the centre of the bole of the tree to the staff of the dendrometer. At this point (where the operator stands) the staff of the dendrometer is to be fixed in the ground. Then setting the instrument in the direction of the tree turn the square face of the instrument (which works on a pivot fixed at the upper angle) until the plumb line falls direct upon the line A B (Fig. 370). Fix the square in this position by the clamp •crew, and then look through the " sight" (the perforation running through the square from C to A,Fig. 370), and mark the place on the tree where the line of sight cuts the tree, as at B in Fig. 371. This point (b) will give the level corresponding to the height of the observer. Next loosen the clamp screw and turn the square until the line of sight cuts the extreme top of the tree, then tighten the clamp screw again.
The plumb line will then be seen to make a triangle with the base and altitude lines, as shown in Fig. 370. The height of the tree will be indicated by the numberof the line of altitude, which is intersected by the plumb line, on the base line corresponding with the measured distance from the tree.
The diagram (Fig. 370) shows clearly what takes place during an observation. Suppose the base line from the centre of the tree trunk to the observer measures 50 ft., and after " sighting " the top of the tree the plumb line falls over the square in the manner indicated in the . diagram (the upper figure), the height of the tree measure! would then be 25 ft. Again, if the b:ise line measured 100 ft., and after "sighting" the topmost point of a tree, the plumb line fell across the square, as in the lower figure in the diagram, the tree would be 50 ft. in height. Of course, in every case the height from the ground to the observer's eye must be added to the height read on the instrument.
In measuring reclining trees or other objects, care must be taken not to measure the base line from the centre of the tree trunk, but from the point on the ground perpendicular to the highest part of the tree. This point may be ascertained by holding a plumb-line between the eye and the tree, and marking on the ground the place thus indicated, as at B, Fig. 372. On finding this point perpendicular to the highest part of the tree, the observer may proceed as in the preceding instructions.
It will thus be seen that in measuring objects not exactly perpendicular, some care is necessary in the operation, or the measurements will be inaccurate. In the case of ascertaining the height of an object, as for instance that represented in Fig. 372, if the base line were measured from the centre of the bole, instead of from the point B, the observed height would be too great. In short, if the base line were measured from the centre of the bole on the side to which the tree is leaning, it would give too great a height, and on the other hand, if the base line were measured on the side the tree is leaning from, the height so ascertained would be less than the true height of the tree.
In measuring the height of round or flat-topped trees, the observer must choose a station sufficiently distant, so as to fully see the highest part. If viewed too near, as at A in Fig. 373, it is impossible for one to sea the highest part of the tree, and the result is that the height is greatly increased. Therefore, in order to avoid such errors, the object should be viewed as far back as possible, so as to obtain a view of the highest point right over the true perpendicular, or, in the event of- this not being possible, the perpendicular anil height of some definite point may be ascertained as in Fig. 372.
The height of any part of a tree or other object may be ascertained by subtracting the result of one observation from that of another.
This instrument possesses many advantages. It is simple, no calculation being required; the height of any tree or other object can be ascertained at any convenient distance, and by it the height of any portion of a tree, such as the height of the trunk, can be ascertained from one station. It is, moreover, light and portable, not its least recommendation for an instrument of this kind.