or 2'5344 inches to a mile or o'3946 mile to an inch.

or 3.168 inches to a mile or 0.3157 mile to an inch.

or 4224 inches to a mile or 02367 mile to an inch.

or 6.336 inches to a mile or O1578 mile to an inch.

or 8.333 feet to an inch or o'12 inches to a foot.

Scale

or 4166 feet to an inch or 0.24 inches to a foot.

50

The scales for distances on sections are identical with, or bear some convenient ratio to, those used on the corresponding plans. The scales for heights on sections are generally arranged to be 10 times the scale for distances, as this is often most convenient; another method is to use 10 times the number of chains in horizontal distances per feet in vertical; thus for 6 chains horizontal, 60 feet vertical, and so on; but the two scales of 100 feet to an inch, and 40 feet to an inch, are the minimum vertical scales laid down by Parliamentary Standing Orders for long and cross sections of proposed works with alterations of existing works. Records of height or depth made on plans are generally in feet and decimals, mètres and decimals, or fathoms and quarters. The scales principally used in the Indian Surveys are:

4 miles to the inch.

Triangulation charts G. T. S. Indian Atlas sheets. Revenue survey of Northern India.

G

2 miles to the inch.

I mile to the inch.

Maps giving heights, G.T.S. Maps of coal-fields and mineral districts. Topographical surveys of certain provinces. Revenue maps of Southern

India.

Besides these there are maps from Surveys of Rivers on various scales, 2 miles, I mile, mile, 1000 yards and 500 yards, to the inch being most common; also railway maps or plans, which show greater variety as to scale.

Section 2. CHAIN SURVEYS.

General Remarks.-Work of this description, requiring the fewest and least expensive appliances, and the smallest amount of thought and knowledge, possesses the disadvantage of being monotonous and laborious. It may be correctly applied to portions of land, estates, or plots, not exceeding one square mile at a time; when used for larger tracts, the portions thus surveyed should be fitted on to, reduced, and verified by the skeleton work or distant points of some larger survey, such as the Great Trigonometrical or large Topographical survey of the country.

In estate-surveys, which are mostly surveyed with chain alone, the acreage of various holdings is often a more important consideration than the plan, hence the chain used for them is the 66 feet or Gunter's chain of 100 links, a convenient submultiple of the acre; the distances are measured in such chains and links to the entire exclusion of feet, and the acreage can then be more rapidly calculated.

In other surveys, in which acreage is comparatively

unimportant, and the introduction of Gunter's units might be troublesome, either the 100 feet or the 50 feet chain is used, and all distances are measured in feet; any mileage required on any part of the plan being subsequently marked or scaled off.

The appliances required on this work are:

First. The chain itself. (See appliances for measuring distance, Chapter I., pages 6 to 8.)-These are made of various patterns, and are galvanised, painted, or plain. The latter are most liable to rust, while the first are liable to be less correct from the additional process they undergo. The long-linked pattern, having each link a foot or a Gunter's link in length, is considered advantageous, from being lighter with the same amount of similar material, from an idea that its form shows most readily any accidental kink or derangement during chaining, and from its links being easily hammered straight after being accidentally bent. The curb-chain and such smalllinked patterns are modifications introduced with the use of superior metal, the amount of which is correspondingly reduced; they can be made excessively light and convenient, but when damaged are less easily rectified than the old pattern. In any case the length. of a new chain should not only be tested with a good standard when simply laid straight, but also again after stretching it by a weight. During the progress of survey work, the length of the chain should be daily tested by comparison with a temporary standard marked for the purpose and kept invariable. If a chain has to be mended or altered, the time and date should be recorded in the field-book of the survey.

The pins or skewers used with the chain should have numbered tallies attached to them; and the tallies at

every 10 feet or 10 links on the chain should read from the beginning to the end of the chain, not from both ends to the middle. The pins should be strung in order on a broad strap.

The other appliances necessary are an optical square or a cross staff for setting out right angles, a measuring tape for measuring long offsets, a painted offset staff for short offsets, and a set of ranging rods for marking stations. (See Demarcation of Survey Points, page 67.)

The optical square consists of two small silvered. mirrors fixed in a brass box at a permanent inclination to each other of 45°, thus reflecting any object through an angle of 90°, the unsilvered portion of one mirror giving a direct view of the object, while the reflected and the direct object can be exactly superimposed in the field of view of the observer, when they are at right angles to each other. It requires occasional testing and adjustment, which is effected by a screw moving one mirror very slightly.

The cross staff, or cross head, has at least two pairs of sights fixed at right angles to each other on the upper end of a staff about five feet long, shod with iron; but generally these heads have four pairs of sights, which then enable half a right angle to be set out when required. They are made of various patterns; those least liable to damage and the lightest are preferable.

Either of these appliances may be easily tested by setting out four right angles from one spot.

The measuring tape and the offset staff, which is painted black and white in alternate lengths, should both be subdivided in accordance with the special unit adopted in the survey, either in feet or Gunter's links, but not with both of them, as that might cause mistakes.