218 



occupied by the measured 6 feet, as compared with the space occupied by the 

 whule tree gives the height. For instance, iu the 1 ist oak tree he had 

 measured, the 6 feet exactly occupied 4-lGths, that is one quarter of an inch, 

 on the scale, and the wLole tree occupied exactly 2g inches, jast ten times 

 as much, which gave at once 60 feet as the height of the tree. They would 

 observe that by this simple plaa it mattered not that the ground should be 

 level, so long as you could see the whole tree— an immense advantage over 

 most of the plans he mentioned already— and it signified not the more that 

 the tree should stand perfectly upright, for if the tiee sloped you had only 

 to slope tho scale in the same degree to get the length of the tree, and on 

 this point it might be said to be superior 'o the more scientific methods he was 

 abuut to mention, which only gave the height of the highest point from the 

 ground below it (hear, bear). He was afraid, however, that he covild not say 

 much for the originality of this plan. It had doubtless occurred to one or 

 two other people (a laugo) and they would remember in Thucydides that the 

 height of the walls of Pl.itrei, was ascertained by the knowledge of the thick- 

 ness of a single brick, and counting the number of courses of bricks to the top. 

 It rnust be confessed also that this plan does require a very steady hand 

 and a very tteady eye, and in fact that its accuracy must depend so much on 

 the observer himself, that he could only claim for it generally a close approxi- 

 mation to accuracy. 



The remaining plans he would bring before them, all depend on the 

 angle of elevation of the top of the tree being carefully taken. 



The sixth, or the Carpenter's plan, was the most sim|)le of these. He 

 takes his "mitre square," and moves it nearer or further from the tree, 

 until whilst the square is held true, as shown by its plummet line, he can 

 look along the angle (che diagonal of the square, and therefore 46°), and just 

 see the top of the tree. In order to see this, he knows that he must of 

 necessity be placed at exactly the same distance from the tree, as the top of 

 the tree is from the ground, or in other words, since the base and perpendicular 

 sides of his mitre-square»are exactly tqual, the same angle extended will 

 not be true unless both the samy sides continue to be of equal length. He 

 has then only to measure the distance from the tree, and add to it the height 

 from the ground to his square to give the true height. This is a very ready 

 excellent plan, but it also requires that the ground should be level or nearly 

 so, and that neither bushes nor any other obstacle should exist at the exact 

 spot required for the observation (hear, hear). 



The seventh plan is to take the angle of elevation by the Quadrant in 

 the ordinary way, and to measure the distance of the tree from the spot 

 where the angle is taken, to transfer it to paper, and work out its height 

 by a secondary calculation from the exact angle. He would not dwell upon 

 this, because it was rather too troublesome to ordinary observers j it was, 

 moreover, the exact method usually followed, therefore well known. 



