« ΠροηγούμενηΣυνέχεια »
10 links. Answer.
2. How many links in a division, if a map be laid down by a scale of 10 perches to an inch?
12.5 or 121 links. Answer.
And so of
To protract a field-book, taken by the angles of the field.
Note. We here suppose the land surveyed is kept on the right hand as you survey.
Draw a blank line with a ruler of a length greater than the diameter of the protractor; pitch upon any convenient point therein, to which apply the centre-hole of your protractor with your pin, turning the are upwards if the angle be less than 180, and downwards if more ; and observe to keep the upperedge of the scale, or 180 and 0 degrees upon, the line : then prick off the nunber of degrees contained in the given angle, and draw a line from the first point through the point at the degrees; upon which lay the stationary distance. Let this line be lengthened forwards and backwards, keeping your first station to the right, and second to the left;
and lay the centre of your protractor over the second station, with your pin, turning the arc upwards, if the angle be less than 180, and downwards, if more; and keeping the 180 and 0 degrees on the line, prick off the number of degrees contained in the given angle, and through that point and the last station draw a line, on which lay the stationary distance; and in like manner proceed through the whole.
In all protractions, if the end of the last station falls exactly in the point you began at, the fieldwork and protraction are truly taken, and performed ; if not, an error must have been committed in one of them : in such case make a second
protraction; if this agrees with the former, and neither meet nor close, the fault is in the field-work, and not in the protraction ; and then a re-survey must be taken.
The accuracy of geometrical and trigonometrical mensuration, depends in a great degree on the exactness and perfection of the instruments made use of; if these are defective in construction, or difficult in use, the surveyor will either be subject to error, or embarrassed with continual obstacles. If the adjustments, by which they are to be rendered fit for observation, be troublesome and inconvenient, they will be taken upon trust, and the instrument will be used without examination, and thus subject the surveyor to errors, that he can neither account for, nor correct.
In the present state of science, it may be laid down as a maxim, that every instrument should be so contrived, that the observer may easily examine and rectify 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 ; Anastronomical quadrant,or circular instrument.
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 used for 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.
OF HEIGHTS AND DISTANCES.
1st. Of Heights.
Pl. 5. fig. 18.
THE instrument of least expence for taking heights, is a quadrant, divided into ninety equal parts or degrees; and those may be subdivided into halves, quarters, or eighths, according to the radius, or size of the instrument: its construction will be evident by the scheme thereof.
From the centre of the quadrant let a plummet be suspended by a horse hair: 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 been