صور الصفحة
النشر الإلكتروني

Having thus related to you our success, I shall now endeavour briefly to describe the apparatus by which it is effected.

The boiler is placed in the back shed, and is made of copper, and weighs 80 lbs. with the taps included. It is furnished with a safety valve, a tap and funnel at the top to pour in the water, a tap at the bottom to let out the water, another to show when there is water in sufficient to allow room for steam to be generated, and a tap at the top with a small pipe attached which nearly reaches down to the bottom of the boiler. This last is to show when the water is too low, which it does by permitting steam to escape, which would not be the case so long as the end of the attached small pipe was covered with water.

The pipe thai conveys the steam into the houses is 1} in. diameter; it rises from the boiler 3 ft., and is then carried through the back wall down to the floor inside under the back stage; it then branches right and left to each end of the houses; is then led across each end, and on the front; the branches on the ends and front being reduced to 1-inch pipe. In those pipes, and also on the back, are holes drilled every 6 ft., into which holes small pipes 6 in. long are screwed. These small short pipes are in the form of the letter T, to throw the steam horizontally, so as to diffuse it through the air before it reaches the pots or plants.

We soon found the small pipes on the main back one were not necessary, as the steam spread itself from the front and ends quite sufficiently for our purpose. The cost of fuel for this apparatus is but trifling, and the steam and the pipe that contain it heat the houses so much, that a very considerable saving of coals in the hot-water boilers is the consequence. We have therefore attained two objects ; the supplying of moisture to the internal atmosphere most effectually, and a saving of fuel.

The boiler and pipes and fitting up cost about 131.; but, if we had had an iron boiler instead of copper, it would not have cost more than 101.

The question now is, whether the above method is worthy of imitation. All I can say on the matter is, that both Mr. Brocklehurst (my spirited employer) and myself are perfectly satisfied with it, and I shall be happy to give any further information to you or any of your readers who may require it.

Fence, Macclesfield, Cheshire, Feb. 15. 1840.

Art. IV. Description of the Hypsometer, an Instrument invented by

John Sang, Esq., Land-Surveyor, for taking the Heights of Trees, Buildings, and other objects. Communicated by Mr. Sang, Land-Surveyor, Kirkcaldy.

I have taken the first leisure hour to make you the instrument for measuring the height of trees and buildings which I men


tioned to you when having the pleasure of visiting you at Bayswater. It is sent by post at the same time as this letter.

The instrument was tried on some houses and trees here, and it gave their height (especially the houses) with great accuracy. It is rather difficult to manage at first, but after a few trials it becomes quite easy. The method is as follows:

By means of a small hook (if a knot of white cloth be attached to it, so much the better), fix the end of a tape line to the bole of the tree, at exactly the height of the observer's eye from the ground. Retire from the tree, letting the tape line unwind until

, by using the instrument, the top of the tree and the end of the tape line are seen quite close together. Add the height of the observer's eye to the length of the tape line, and the sum is the height of the tree. Now, the difficulty is, to catch the image of

. the top of the tree in the instrument, and it is this which requires a few trials, although any person who has been accustomed to use a sextant will do it at the


first. Hold the instrument by one of the milled ends, taking care that the fingers do not project over any of the holes, and that the brim of the hat is out of the way. Apply the eye to the round hole marked a in fig. 31., and look through in the direction of the small square hole b, the instrument being held so that the line joining a b is about level, while the large square hole c is turned toward the sky. You will then see some object directly through the small hole, and at the same time the image of some other object, the light from which enters the large aperture, and, after being reflected by the two mirrors inside, passes into the eye. Whatever two objects are thus seen in contact, subtend at the eye an angle of 45°, as in

fiy. S2. ; so that, if
one of them be the 1
end of the tape
line on a level, or nearly so, with the
observer's eye, while the other is the top
of a tree, supposed to be growing
straight up, the distance from the eye
to the bole of the tree will be exactly
equal to the distance from the end of

the tape line to the top of the tree. You will thus observe that the accuracy of the measurement depends on the tree being erect from the ground. On sloping ground the measurer would require to go out from the tree in

[ocr errors]


such a direction that the tape line was perpendicular to the stem, but this could be judged sufficiently well by the eye to give the height, if 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 upright on a level courtyard.

The principle of the instrument is quite simple, being exactly the same as that of the sextant or quadrant, only that the mirrors are fixed at a certain angle instead of being movable. Thus, in fig. 33., a is the eye, b a mirror partly

33 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 c b, and again reflected from the mirror b in the direction b a to the eye; at the same time another ray of light comes from an object o direct to the eye at a, without being reflected. From the nature 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 height. 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 63° 26' 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 observer's eye. (See fig. 34.) Of course a small deviation from squareness in the trees and tape line will make a greater error than with the instrument sent, but still it will give a result near enough for all practical purposes.

I have only to add, that the mirrors are made of common window glass selected as the most even from among a great many pieces, but still they are not quite flat. I had some glass from London perfectly true and fat, but so dim and badly polished as to be unfit for use.

Kirkcaldy, Jan. 31. 1840.


[ocr errors]

Postscript in Answer to some Questions asked of Mr. Sang by the

Conductor. The instrument for measuring the height of trees is not a pocket 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-measurer, are all varieties or improvements on Hadley's first invention. The two latter differ from the rest in the mirrors being permanently fixed at angles suitable for the purposes for 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 it is so exceedingly simple, that I have no doubt the idea of modifying the sextant, so as to make it readily measure the height of trees, has 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 the 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 here to make them, and they might be sent from the seedshop (see p. 93.] to any place by post.

Kirkcaldy, Feb. 10. 1840.

Art. V. On Wire Field Gates. By George Buist. A SINGULARLY light, strong, and unexpensive variety of field and fancy gates has of late been introduced into some of the midland counties of Scotland. They are the contrivance of Mr. George Buist, late of Fifeshire, now of Bombay. They are made wholly of iron, the frame being of light bars, the filling up of wire or small rods. The principle on which they are constructed is, that all the portions requisite for filling up the gate, and which generally only add to its weight and cumbrousness, shall be made to act as stays or strengtheners to brace up the frame; and that these, being all exposed to a longitudinal strain alone, shall be made of very small iron rods, or strong wires, which, when exposed to nearly the direction of their greatest strength, will sustain without injury four or five times the pull to which, in gates, they have ever any chance of being exposed. Of the simplest and cheapest of these, which is merely the skeleton of the more expensive and complex one, the adjoining is a representation.

Fig. 35. represents a gate 9 ft. long, and 13 ft. in height. The framing (a b c d) is of light bar iron, put together in the ordinary way; e f, g h, k l are straps of light iron fastened at the extremi

[merged small][ocr errors][merged small][ocr errors][ocr errors][merged small]

ties by rivets to the upper and lower rails, which they, amongst other things, serve to tie together. The whole of the other fillings up consist of wire or rod iron, about the thickness of a goose quill. ac, db are diagonals for the purpose of maintaining the gate in a rectangular form. a q b is a back stay or brace for a b, to prevent it from bending downwards by pressure. It passes through punched holes in the straps e f g h, and k l, so that a b is stiffened by it at the three points e, g, and k. In like manner, drc is a back stay similarly arranged in reference to d c, to prevent it from yielding upwards. The three straps, ef, gh, and k l, connect the two rods together, and give each of them the supporting power of both back stays either way: that is, ab, as already shown, is prevented yielding downwards by its own proper back stay a qb, and it is prevented yielding upwards by the back stay of dc, that is, d r c; and so with d €, whose depression is prevented by the stay a q b, to which it is tied by the straps. So, in like manner, with reference to the ends of the gate, the stays a t d and b s c come into operation. Now these, when combined as in fig. 35., give a system of universal bracing, such as that the framing, strengthened by them, can be disshaped by no force short of one of sufficient power to lengthen out the wires longitudinally. They form a gate, not adequate, certainly, for the retention of lambs or pigs without some additional wires, but which will keep in cattle of every sort, its largest apertures being only a triangle 15 in. by 9 in. Any sort of lock or hanging may be used which seems expedient. When a gate is suddenly shut, it will be observed to tremble and vibrate violently for some seconds, at the fore part or lower part farthest from ihe crook end. This, like every other kind of concussion, is of course injurious to the structure of the gate.

« السابقةمتابعة »