SURVEYING 491 edge against a fine needle stuck up in the board, and is then directed successively toward the corners of the tract to be surveyed and any other prominent objects, toward which from the needle lines are to be drawn on the paper. One of these lines should also be in the direc- tion of the measured line. The instrument is then taken to the other end of the measured line, the needle is removed along the last line named on the board a distance corresponding, according to the scale adopted, to that of the measured line on the ground, and the board is so placed as to make the line toward the for- mer station correct. The ruler is then again pointed to the same objects, and lines are drawn toward each from the new position of the needle. Their intersections with the for- mer lines designate the places of these objects on the plane. The plane table is used in vari- ous other ways, as by moving it from one cor- ner to the next, and placing it at each so that the last line drawn coincides with that in the ground. From any central point also radia- ting lines may be measured to the corners, and the distances measured and marked off accord- ing to the proper scale. Eivers, brooks, and roads are surveyed by measuring a succession of lines following their general course, and ta- king offsets from the sides of the line. Streets are followed in a similar manner. Distances are sometimes measured upon roads, where expedition is more important than extreme accuracy, by various substitutes for the chain, some of which, as the odometer and pedom- eter, have been noticed under the former head. One may soon accustom himself to. pace in straight lines, and with steps of uniform lengths, the most exact method being to regu- late the natural step, rather than to try to at- tain one of any determinate length. The usual average step" of a man is that of the English military pace, 2 ft. The French geographical engineers accustom themselves to take regular steps of yV of a metre, or 2 ft. 7$ in. The field work being completed, the figure of the tract surveyed is reproduced upon a diminished scale by what is termed plotting; and from this plot the contents are ascertained by a series of mathematical calculations applied succes- sively to the several divisions, or by the meth- od of calculation of latitudes and departures, for which a table of natural sines is required, unless " traverse tables " giving the latitude and departure for any bearing, as furnished in some books on surveying, are at hand. An approximate estimate of the number of acres included in the survey is sometimes made by drawing the plan upon sheet lead of uniform thickness, or upon Bristol board or heavy pa- per, cutting out the piece on the boundary lines, and weighing it in a delicate balance. The weight may then be compared with that of a similar piece that exactly comprises a defi- nite number of acres, laid out upon the same scale. The extensive territories of the United States are surveyed upon a peculiar system, planned with reference to the division of the lands into squares of uniform size, so arranged that any tract of 160 acres, or a " quarter sec- tion," may have its distinct designation and be readily found upon the map or recognized upon the ground by the marks left by the surveyors. Each great survey is based upon a meridian line run due N. and S. by astronomical mea- surements the whole extent of the survey in these directions ; and upon a " standard paral- lel" or base line running E. and W., similarly established with great accuracy. Parallels to these lines are run every 6 m., usually with the solar compass corrected by frequent celestial observations ; and thus, as nearly as the figure of the earth admits, the surface is divided into squares of 6 m. N. and S. and the same E. and W., each one containing 36 sq. m. or sections, into which the territory is further divided by meridians and parallels run at every mile ; while the half mile being marked on these lines by setting what is called a quarter post, the points are established for the subdivision into quarter sections. The .squares of 36 sq. m. are termed townships, often contracted to "towns;" and each line of them E. and W. is numbered either N. or S. from the base line, and each line of them K. and S. is termed a range and is numbered E. or "W. from the meridian. The N. and S. lines bordering the townships are known as range lines, and the E. and W. as township lines. Each survey is designated by the meridian on which it is based, and of these principal meridians there are 6 designated by numbers and 18 by special names. The following table, compiled from information furnished from the general land office in August, 1875, gives the designation of the meridians, their longitude W. from Greenwich, and the 1ST. latitude or other de- scription of the principal base lines : DESIGNATION OF PKINCIPAL MEKIDIAN. Longitude W. ft^m Greenwich. Principal base lines, N. lat. First, boundary bet. 84 51' Second, through In- 86" 28' ___% , ,' ' , _,,. 89 10' 80" Fourth.N.from mouth of Illinois river Fifth, N. from mouth of Arkansas river.. Sixth 90" 29' 56" 90 58' 97 22' 88 58' 12" Mouth of St. Francis E. 40 Michigan 84" 19' 9" 7 m. N. of Detroit. Tallahassee St Stephen. .... 84 18' 88 2' Tallahassee, Fla. 81 Huntsville Choctaw 86 31' 89 10' 30" 91 5' N. boundarv of Alabama. 29 m. S. of Jackson, Miss. 81 St. Helena, S, from base 90 11' 81 Louisiana New Mexico 92 20' 106 52' 9" 81 84 19' Great Salt .Lake . . . Boise Mt. Diablo San Bernardino. . . . Humboldt 111" 58' 47" 11 6 20' 121 54' 116 56' 124 11' 40 46' 4" 43 26' 37 53' 84 6' 40 25' 80" Willamette Montana Gila and Salt river. . . 122 44' 111 38' 112 15' 46" 97 15' 56" 45 30' 45 46' 27" 83 22' 57" 34 31'
