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so contrived, that the observer may easily examine and rectity the principal parts; for however careful the instrument-maker may be, however perfect the execution thereof, it is not possible that any instrument should long remain accurately fixed in the position in which it came out of the maker's hand, and therefore the principal parts should be moveable, to be rectified occasionally by the ob
AN ENUMERATION OF INSTRUMENTS USEFUL TO
A SURVEYOR; Fewer or more of which will be wanted, according to the extent of his work, and the accuracy required.
A case of good pocket instruments.
To be added for county and marine surveying ;
A good refracting and reflecting telescope.
For marine surveying ;
A station pointer.
Besides these, a number of measuring rods, iron pins, or arrows, &c. will be found very convenient, and two or three offset staves, which are straight pieces of wood, six feet seven inches long, and about an inch and a quarter square ; they should be accurately divided into ten equal parts, each of which will be equal to one link. These are usedfor measuring offsets, and to examine and adjust the chain.
Five or six staves of about five feet in length, and one inch and an half in diameter, the upper part painted white, the lower end shod with iron, to be struck into the ground as marks.
Twenty or more iron arrows, ten of which are always wanted to use with the chain, to count the number of links, and preserve the direction of the chain, so that the distance measured may be really in a straight line.
The pocket measuring tapes, in leather boxes, are often very convenient and useful. They are made to the different lengths of one, two, three, four poles, or sixty-six feet and 100 feet ; divided, on one side into feet and inches, and on the other into links of the chain. Instead of the latter, are sometimes placed the centesimals of a yard, or three feet into 100 equal parts.
From the centre of the quadrant let a plummet be suspended by a horse bair : or a fine silk thread of such a length that it may vibrate freely, near the edge of its arc: by looking along the edge AC, to the top of the object whose height is required ; and holding it perpendicular, so that the plummet may neither swing from it, nor lie on it ; the degree then cut by the hair, or thread, will be the angle of altitude required.
If the quadrant be fixed upon a ball and socket on the three-legged staff, and if the stem from the ball be turned into the notch of the socket, so as to bring the instrument into a perpendicular position, the angle of altitude by this means, can be acquired with much greater certainty.
An angle of altitude may be also taken by any of the instruments used in surveying; as has bee
particularly shown in treating of their description and uses.
Most quadrants have a pair of sights fixed on the edge AC, with small circular holes in them; which are useful in taking the sun's altitude, requisite to be known in many astronomical cases ; this is effected by letting the sun's ray, which passes through the upper sight, fall upon the hole in the lower one; and the degree then cut by the thread, will be the angle of the sun's altitude ; but those sights are useless for our present purpose, for looking along the quadrant's edge to the top of the ob ject will be sufficient, as before.
PL. 5. fig. 19.
To find the height of a perpendicular object at one station,
which is on an horizontal plane.
of the steeple, or the base, 85 feet. Height of the instrument, or of the ob
server, 5 feet.
Required, the height of the steeple.
The figure is constructed and wrought, in all respects, as case 2. of right-angled trigonometry ; only there must be a line drawn parallel to, and beneath A B of 5 feet for the obseryer's beight, to represent the plane upon which the object stands ;