37= 



SCIENCE OK AGRICULTURE. 



Part II. 



tl 



to regulate the direetiou 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 pens, one iron pin 

 with inchei and halves marked regularly upon the (idea of it from the top downwards After having driven 

 in the tirst wooden peg at the point whence you mean to conduct the drain, and having retted the one 



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

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

 that the top of il may l>e half an inch lower than that of the iron pin. Place the leg of the frame again 

 upon tins 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 as far as you mean to carry the 

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

 it is useful in ascertaining the proper descent along the liottom 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 

 become! a a, or delta. 



250'i. The. si/uare level (Jig- 235. b), is made of several pieces ; the usual length 



generally five feet and a halt', and the 



height lour feet, or four feet ami 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 level now in use, as being equally 



accurate in ascertaining the relative 



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." 



2503. The object staff (fig- 235. 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 white. 



2504. The levelling si'iff is composed of two pieces (Jig. 235. 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 extended, 

 a rod of ten feet. They have a line of feet graduated into hundredth parts. The 

 index (f) 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 the fiducial edge (g). 



•2505. The meiisuring-chain, mensuring-rod, pocket-rule, poles for setting out straight 

 lines, stakes for driving in at fixed points, and a variety of other instruments, and their 

 appendages, are occasionally required by the agriculturist who lays out estates, or effects 

 territorial improvements : but these, not being strictly agricultural implements, do not 

 require to be described. 



2506. The odometer (odos, a way, and melreb, to measure) is a very ingenious instrument, invented in 

 1821 by Mr. Hunter.ot Thurston in Scotland, who has given the following description of it to the Highland 

 Society. The wheel a {Jig. 2:36.) is made of light iron, and measures two yards in circumference, being 

 divided by six spokes into feet. One spoke must 

 be painted white. The handle is divided at c, like —^-~ 



a fork, and embraces each end of the axis by its 

 elasticity. Through the axis is a hole into which 

 the end h of the way-wiser fits, and is held fast by 

 a nut </. The way-wiser [Jig. 237.) consists of a 

 frame fg; /being hollow to receive a perpetual 

 screw A, a part of which is visible near the index m. 

 At the other end of the screw is a nut ;', which 

 keeps it in its place. The screw turns two brass 

 concentric cogged wheels k and/,- k conceals the 

 scale of/, except where a piece is cutout, leaving an 

 index at the beginning of the scale of /,, and « hich 

 in the drawing points to 7^ ofi The scale of k is 

 numbered towards the left, and that of / to the right. 

 The wheel k has 100 cogs or teeth, and 1 101 ; conse- 

 quently) as the same endless screw turns both 

 wheels, it is evident, that when k has made a com- 

 plete revolution of 100 teeth, / will also have made a 

 revolution of KM) teeth ; and the index of A- will point 

 to 1 of/, because i has 101 teeth After a second 

 revolution it will point to 2, and soon ; the number 

 it points to marking the number of revolutions ; each revolution showing 100 turns of the iron wheel 

 ii. Accordingly, a measures 6 feet, or 1 turn ; k lt.O tim>>s 6 feet, or 600 feet, or 1 revolution ; and / 101 

 times BOO feet, or 60,600 feet, equal to nearly II' English miles, the range of the instrument : 88 turns 

 of this wheel make a mile. It is advisable always to commence with the way-wiser set at or zero ; to 

 do this, take out the screw in the centre, when the brass wheels /■■ and / can both be set at zero, and 

 the screw replaced. Set the wheel a upon the ground with the white spoke undermost, and fix the way- 

 wiser into tne wheel by means of the nut d, always observing to put it on the left side, as shown in the 

 plate at e. At any period of measuring you can tell exactly how far you have gone, and proceed without 

 again setting the way-wiser at 0. Suppose, as in the figure, the spoke No. 2 at the ground, the index 

 m pointing at 26 of k, and the index of k pointing at 78 of / ; then the distance measured is 7826 turns 

 of a and two feet ; and as a measures two yards, 7826 x 2 = 151659 yards, to which add the two feet. 

 In reading off, particular care must be taken always to read the large figures viz those on the wheel /) 

 tirst, and afterwards to add thesmall figures(viz. tho*e on the wheel k^ ; and, if the figures on k amount to 



