366 



SCIENCE OF AGRICULTURE. 



Part II. 



found iH the road or common levels (Jig. 259.), water level, the triangular and the 

 square level. 



259 



S=4. 



2422. The ivater level is that which shews the horizontal line by means of a surface of 

 water or other fluid ; founded on this principle, that water always places itself level or 

 horizontal. The most simple level of this kind is made of a long wooden trough or 

 canal, which being equally filled with water, its surface shews the line of level. It is also 

 made with two cups, fitted to the two ends of a straight tube, about an inch in diameter, 

 and three or four feet long, by means of which the water communicates from the one cup 

 to the other, and this pipe being moveable on its stand by means of a ball and socket, 

 when the two cups shew equally full of water, their two surfaces mark the line of level. 

 It may also be made with two short cylinders of glass, three or four inches long, fastened 

 at each extremity of the pipe with wax or mastic. The pipe is filled with common or 

 colored water, which shews itself through the cylinders, by means of which the line of 

 level is determined ; the height of the water with respect to the centre of the earth, being 

 always the same in both cylinders. This level is very simple and commodious for level- 

 ling small distances. 



2423. The American or triangular level [fig. 260 a.) is formed of two pieces of thin wood joined by a cross bar, 

 the whole in the form of the letter A. The manner of using it is simply thus : At the place from where the 

 level is to be taken, drive a wooden pegintotheground, close in to the top, upon which one of the legs of the 

 frame or A may rest ; then bringing round the other leg till it touch the ground, there drive in a second peg, 

 turning round the other leg as before ; and where it touches the ground again, drive in another peg, and so 

 on along the whole line to be levelled. Thus, with very little trouble, and with as much accuracy as with tlie 

 finest spirit-level, may the course of a drain be easily ascertained. But as it is necessary that a drain 

 should have as much declivity as to allow the water to run freely, it will be requisite, in taking the level, 

 to regulate the direction of the line accordingly. Half an inch fall, in the length of the frame, will be 

 sufficient For this purpose, it will be expedient to have, besides a number of wooden pegs, one iron pin 

 with inches and halves marked regularly upon the sides of it from the top downwards. After having drove 

 in the first wooden peg at the point from whence you mean to conduct the drain, and having rested the one 

 leg of the frame upon it, turn round the other till it be level with the first peg ; there put in the iron pin, 

 so that this leg of the frame may rest on the top of it, when level ; then drive in a wooden peg so far, as 

 that the top of it may be half an inch lower than that of the iron pin. Place the leg of the frame again 

 upon this second peg, turn it round to a level, putting in the iron pin till the top of it be equal with the 

 foot of the frame ; then drive in another wooden peg close by the side of it, till the top of the wooden one 

 be half an inch lower than that of the iron pin. Proceed in this manner so far as you mean to carry the 

 drain, which will have the same degree of declivity all the way along. When made on a smaller scale, 

 it is useful in ascertaining the proper descent along the bottom of a drain, while the workmen are laying 

 it ; but when made for this purpose, the cross-bar must be fixed to the bottom of the legs, so that the A be- 

 comes a A, or delta. 



is made of several pieces; the usual length 

 260 



2424. The square level (Jig. 260 b.), 

 generally five feet and a half, and the 

 height four or four feet and a half. It 

 may be either used like the water level, 

 or the American level. According to 

 Marshal, it has been found " preferable 

 to any other level now in use, as being 

 equally accurate in ascertaining the re- 

 lative heights of distant objects, as in 

 minutely tracing step by step the required 

 line of communication, so as to give every 

 part of it an equal and uniform descent." 



2425. Tlie object staff {Jig. 260 c.) is used with the water or square level : for either 

 it should be exactly of the same height as the level ; the cross piece at top should be a 

 foot or more in length, and three inches broad, painted white on one side for opposing to 

 dark objects, and black on the other for opposing to such as are Avhite. 



2426. The levelling staff is composed of two pieces, {Jig. 260 d, h, and e, e), which slide 

 on each other : they are each of about five feet in length, so as to form, when fully extend- 

 ed, a rod often feet. They have a graduated line of feet into hundredth parts. The 

 index (/) slides firmly on them; and is moved up or down (by signal) by the attendant 

 who carries the staff, till the observer finds it coincide with the intersecting wires of his 

 telescope. Its height on the staff, of course, marks the difference of the level. It has 

 two horizontal and parallel black stripes, which at considerable distances are of use to 

 direct the eye more readily to tlie fiducial edge (g). 



