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These tabular forms can of course be varied to suit different localities, where different instruments are used, or where other periods of observation may be preferred. Those given above are recommended by Sir J. Herschel where only three daily readings are recorded.
The barometer should be fixed in a good light, but one not exposed to the rays of the sun, drafts of wind, or very sudden changes of temperature. Its actual reading should always be recorded, leaving all corrections, for index errors, temperature of the mercury (for which a column is provided), capacity, &c., to be subsequently applied. The thermometer should be hung for observation out of doors, in a perfectly-shaded situation, but otherwise fully exposed, care being taken that it is not so placed as to be affected by reflected rays from water, buildings, or light-coloured hard soil, or by radiated heat from its proximity to the ground. The self-registering thermometer should be fastened so as to admit of one end being detached and lifted up, to allow the indexes to slide down to the extremities of the fluid columns, which is better than using a magnet for the purpose. These instruments are unfortunately very liable to get out of order.
DESCRIPTION OF THE PEDIOMETER AND
These instruments are used for determining the areas of Plans without calculation—whereby a saving is effected of more than half the time consumed in computation, and the liability to error is
very materially diminished.
The instrument consists of a square, and a graduated scale, constructed for three chains to the inch.
a—The milled head, by turning which, motion is given to the brass slider B, and the two pointers R and W.
I–The index to be placed in coincidence with the – division upon the scale.
When the brass slider B is in contact with A, I coinciding with division, and R and W pointing to 0 upon their respective
O scales, the instrument is in adjustment.
When deranged, restore it, by opening R and W to the proper distance, and then moving A and I, the former into contact with B, and the latter into coincidence with
Required the content of the trapezium ECFD.
Ist. -Place the edge A upon the point E, and open B to the point F.
2nd.—Press the square firmly down with the right hand, and with the left place the scale against the edge of it, as shown in the figure.
3rd.—Now press the scale firmly, and slide the square up, until the edge A B is upon the point C.
4th.-Press the square firmly, and slide the scale against its edge until – coincides with I.
Finally.--Press the scale and slide the square down until the edge A B is upon the point D, and taking out the numbers to which W and R point, subtract the latter from the former, and the contents in acres and decimal parts of an acre will at once
The red pointer directs to the numbers that are to be taken from the red scale, and the white one, to those upon the white scale.
When the pointers fall exactly upon the line engraved on the ivory edge of the scale, the folding leaf is to be doubled down to the left hand; but when the pointers fall between any two of the lines on the ivory edge, the folding leaf must then be doubled over to the right hand before the numbers are read off.
For instance, when the leaf is turned to the left and the red pointer falls between the two lines which refer to .008 and .013, turn the folding leaf to the right hand, and the pointer will read 0•10.
It will be found most convenient and most accurate in practice to take the shortest diagonal for the line E F.
THE COMPUTING SCALE,
This instrument answers the same purpose of giving mechanically the contents of enclosures as the Pediometer, but is more simple in its construction and principle of operation.
It consists of a scale divided for its whole length from the zero point into divisions, each representing 2} chains, and is used with a sheet of transparent tracing paper, ruled with parallel lines at equidistant intervals of one chain.
The slider B, which moves along the scale, has a wire drawn across its centre at right angles to its line of motion; and on each side of this wire a distance equal to one of the primary divisions of 2 chains is laid off, and divided into 40 parts. It is evident, then, that during the passage of the slider over one of the divisions of 2} chains, one rood has been measured between two of the parallel lines on the tracing paper; and that one of the smaller divisions would measure between the same parallels one perch. Four of the larger divisions give one acre; and the scale itself, generally made long enough to measure at once five acres, is thus used.
Lay the transparent paper over the enclosure the content of which is required, in such a position that two of the ruled lines shall touch two of the exterior points of the boundaries, as at a and b.
Lay the scale, with the slider set to zero, over the tracing paper, in a direction parallel to the lines, and so placed that the portions c and d are estimated by the eye as equal to each other. Holding the scale steady, move on the sliding frame until the equality of the portions e and f are also estimated. With the slider kept at this mark, move the scale bodily down the space of one of the ruled lines (one chain), and commencing again at the left hand, estimate the equal areas of g and h, sliding the frame on to k and l. When the whole length of the scale, denoting 5 acres, is run out, commence at the right-hand side, and work