than 180, viz. 348. Place your protractor parallel as before, and by the edge of the semicircle with the pin prick at that degree, through which and the end of the foregoing station, draw a blank line, and on it set the distance of that station. In the like manner proceed through the whole, only observe to turn the arc of your protractor down, when the degrees are less than 180. If you lay off the stationary distances by the edge of the protractor, it is necessary to observe, that if your map is to be laid down by a scale of 40 perches to an inch, every division on the pro tractor's edge will be one two-pole chain ; ( a division will be 25 links, and of a division will be 12} links. If your map is to be laid down by a scale of 20 perches to an inch, two divisions will be one twopole chain ; one division will be 25 links; } a division 12; links, and of a division will be 6 links. In general, if 25 links be multiplied by the number of perches to an inch, the map is to be laid down by, and the product be divided by 20 (or which is the same thing, if you cut off one and take the half), you will have the value of one division on the protractor's edge, in links and parts. EXAMPLES. '1. How many links in a division, if a map be laid down by a scale of 8 perches to an inch? 25 210)2010 10 links. Answer. 2. How many links in a division, if a be laid down by a scale of 10 perches to an inch? 25 2|0)250 12.5 or 121 links. Answer: And so of any other. 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 rvey. 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 arc upwards if the angle be less than 180, and downwards if more; and observe to keep the upper edge of the scale, or 180 and 0 degrees upon the line : then prick off the number 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, is 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. REMARKS. 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 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 server 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. for county and marine surveying ; An astronomical quadrant, or circular instrument. А а 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. |