404 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[December, 



the other is the tup of a tree, supposed to be growing straight iij), th 

 distance from the eye to the bole of tlie tree will be exactly equal t 

 the distance from the end of the tape line to the top of the tree. 



I'ib'- I' Fig. 2. 



Fig. 3. 



1 io'- 4. 



You will thus observe that the accuracy of the measurement depends 

 on the tree being erect from the ground. On sloping ground the mea- 

 surer would require to go out from the tree in such a direction that 

 the tape line was perpendicular to the stem, but this could be judged 

 suflBciently well by the eye to give the height, of even a very high 

 tree, nearly correctly. The heights of those houses I tried were given 

 within an inch, which was no doubt owing to their being perfectly up- 

 right on a level court yard. 



The principle of the instrument is quite simple, being exactly the 

 same as tliat of the sextant or quadrant, only that the mirrors are fixed 

 at a certain angle instead of being movable. Thus, in fig. 3, a is tlie 

 eye, k a mirror partly silvered, and c a larger mirror wholly silvered. 

 A ray of light r, falling on the mirror c, is reflected from it in the 

 direction cb, and again reflected from the mirror i in the direction 6 a 

 to the eye ; at the same time another ray of light comes from an ob- 

 ject o direct to the eye at a, without being reflected. From the na- 

 ture of reflected light, the angle r a o is equal to twice the inclination 

 of the mirrors, and is constant, however much the whole instrument 

 may be moved in the plane of the objects, as you will easily perceive 

 by catching the reflection of the candle in the instrument, and moving 

 it in the plane of the milled ends. 



I am sure this very portable instrument will be useful for measuring 

 single trees, or buildings, which are as far asunder as they are high, 

 but I am afraid it will not work well in a close wood, on account of 

 the operator not having room to retire as far from the trees as their 

 heigh;. If this is found to be the case, the remedy is to construct 

 another instrument in which the mirrors are placed so as to give an 

 angle of 03" 2(j' 05". In this case the height of the trees will be 

 equal to twice the length of the tape, added to the height of the ob- 

 server's eye. (See fig. 4.) Of course a small deviation from square- 

 ness iu the trees and tape line will make a greater error than with the 

 instrument sent, bnt still it will give a result near enough for all prac- 

 tical purjjoses. 



I have only to add, that the mirrors are made of common window 

 glass selected as the most even from among a great manv pieces, but 

 still they are not quite flat. I had some glass from London perfectly 

 true and flat, but so dim and badly polished as to be unfit for use. 



Kirlxaldy, Jan. 31, 1S40. 



Poahcript in Aimwer to Home Qiitsli07i8 asked of Mr. Sang by the 

 Conductor. 



The instrument for measuring the height of trees is not a ])ocket 

 sextant, like that of Mr. Blackadder, mentioned in vol. xiv. p. 257, 

 although nearly allied to it. The sextant, quadrant, reflecting circle, 

 improved Wollaston's goniometer, as well as the optical square and 

 tree-measuier, are all varieties or improvements on Hadley's first in- 

 vention. The two latter differ from the rest in the mirrors being per- 

 manently fixed at angles suitable for the purposes fur which they were 

 intended. The pocKet sextant would measure the height of trees 

 quite as well, but, being expensive, and requiring some skill to use it, 

 it is not likely to be much employed for such purposes. There is no 

 sort of merit in designing the instrument; and is so exceedingly sim- 

 ple, that I have no doubt the idea of modifying the sextant, so as to 

 make it readily measure the height of trees, lias occurred to many a 

 one. I, however, never heard of such an instrument, and believe that 

 the one you have is the second of its kind in existence. The other is 

 one which was made for yourself. My father was so much pleased 

 with it that he asked me to make one for him, which turned out neater 

 than the first, and accordingly I sent it to you, as being the better of 

 tlie two. As there is nothing like a Greek name for giving identity 

 to it, you might call it a dendrometer, or, better still, a hypsometer 

 (measure of height). 



Of course any instrument maker could supply these articles; the 

 price, I should think, would be about 20s. each. If there were any 

 prospect of selling a dozen or two, I could easily employ a workman 

 liere to make them, and they might be sent from the seedshop to any 

 place by post. 



Kirkcaldy, Feb. 18, 1840. 



BRITISH MUSEUM. 



Sir — That the British Musenm is a monument — as the French term it, 

 which does honour to this age and country, is what, for peace sake, I will 

 take for granted, notwithstanding that 1 myself perceive nothing par- 

 ticularly monumental or dignified in the sulky and barrack-like aspect 

 of the exterior of the new buildings. Xo one, indeed, can deny that 

 the most frugal economy has been observed there — of course a very 

 plain proof with what rigorous conscientiousness the cash is uniformly 

 doled out of John Bull's public purse. Still there are illnatured grum» 

 biers who opine there are occasions when liberality bespeaks more 

 prudence than cheese-paring economy, and is the more becoming vir- 

 tue of the two; and that such an edifice as the "one I am speaking of, 

 ought to be in every respect a finished piece of architecture. Possi- 

 bly, the fafade — whenever that comes to be executed — may make 

 some amends ; yet it surely would have been better that the whole 

 should be of a piece, and not like Dick Wilson's fine embroidered 

 waistcoat, with its ' back-front' made out of one of his own picture- 

 canvasses. It may be very true that the rest of the building is not 

 intended to be seen, but still as it is not screened from view, it is 

 rather hard to tax the imagination of matter-of-fact folks like myself, 

 so far as to tell us we are to imagine we do not see what is staring us 

 in the face, nor to give credit to our own eyesight. Upon such nota- 

 ble principle of economy, the backs — I mean the East end of St. Paul's, 

 might have been left a plain brick wall ; but it seems Sir Chris- 

 topher's notions of economy were very different indeed from those of 

 Sir Robert. 



I find I have rather committed myself, for wliat I have been saying 

 is likely to call the sincerity of my first sentence, terribly into question. 

 IS'o matter; it can't now be helped; and only proves that liars and 

 critics ought to liave good memories. Accordingly my willingness to 

 ' take for granted ' and so forth, must now either be set down as a pal- 

 pable hnm, or be imputed to my considerate forbearance iu not dis- 

 cussing the architectural merits and demerits of Sir R. Smirke's edi- 

 fice. I will not inquire whether the taste he has shown in the interior 

 of the building is such as to indemnify us for its excessive homeliness 

 without; nor whether he has been prodigal or economical in drawing 

 upon his fancy ami imagination. But I will say that however much 

 he may have consulted convenience rather than splendour, or may havs 

 succeeded in combining both, in other parts of the plan, he has at- 

 tended to neither the one nor the other iu the Reading Rooms, which 

 are about as inconvenient for the purpose as could well have been 

 devised, — to such a degree that without taxing our fancy very much, 

 we might fancy no instructions respecting them had been given to the 

 architect, and that when it was afterwards discovered that the Book- 

 makers and Novel-readers who frequent the British Museum, nuistbe 

 put somewhere, they were accommoda/id where they are now crammed. 

 "Remuneration means five farthing?," and in the present case accom- 



