.428 



MECHANICAL. 



greater number of sides than four, it will 

 be necessary to intersect it with two or 

 more lines, from each of which perpen- 

 diculars must be erected to the angular 

 points along the boundary. Very fre- 

 quently, also, there will be what is termed 

 offsets, or the irregularities of a crooked 

 boundary. In this case it is necessary to 

 take a straight line between two angles of 

 the field where these occur and measure 

 offsets or perpendiculars from that line 

 toward every bend or angle in the boun- 

 dary, by which means a number of small 

 triangles or trapezoids are obtained, to be 

 included in or deducted from the total 

 area, according as they are within or 

 without the boundary line. 



By the aid of a single diagram we 

 shall endeavor to give a few illustrative 

 examples of field work, which will remove 

 •all misapprehension and difficulty. 



Let it be required to find the contents 

 of the triangular field ABC. The field 

 measurements will be the line A B = 900 

 links, or, rather, A F = 300 and F B = 



=600, and the perpendicular F C = 480 ; 

 from these the subsequent operations of 

 plotting and estimating the contents can 

 readily be performed, as will presently be 

 shown. 



We may here remark that the plan 

 observed in keeping the field-book is by 

 no means uniform, nearly every surveyor 

 having his own peculiar method ; and so 

 long as there is method, and the memo- 

 randa be arranged in a way that the 

 measurer can himself fully comprehend 

 them when he leaves the field, it is of 

 small consequence what plan may be 

 adopted. Usually, however, the page of 

 the field-book is divided into three 



columns ; the centre one is occupied by 

 the measurements of the principal lines, 

 and in the right and left hand columns 

 are entered the perpendiculars and offsets 

 from those lines. 



Let us suppose that it is required to 

 find the measurement of a quadrilateral 

 field A B C D. The data to be obtained 

 in the field are these — the diagonal A B, 

 900 links, and the perpendicular F C, 

 480, as formerly, A E, 300 links, and the 

 perpendicular, D E, 510. Again, we will 

 suppose one side of the field has the 

 crooked and irregular boundary, D d e f 

 B; in this case, in addition to those 

 already mentioned, the field measure- 

 ments will be the line, D B, 750 links, 

 distinguished into D a, 150, a b, 270, b c, 

 150, and e B, 180, and the perpendicular 

 offsets, a d, 150, b e, 90, and cf, 180. 



Offsets are usually measured, unless 

 they be of great length, with the cross- 

 staff, which is made 10 links long for this 

 purpose. If, however, these perpendicu- 

 lars exceed two or three lengths of the 

 staff, it is better to use the chain. These 

 offsets, it is also to be noticed, are to be 

 measured in the course of ascertaining 

 the line, D B. 



In simple surveys, such as the pre- 

 ceding examples refer to, it is usual to 

 sketch the form of the field on a page of 

 the book, and enter all the measurements 

 and remarks on the corresponding parts 

 of the sketch. This is an excellent plan, 

 and should always be adopted where the 

 case will admit of it. 



2. We come now to the plotting, or 

 laying down the true figure of the field 

 on paper, according to some determinate 

 scale ; and this operation is no less simple 

 than that of taking the field measure- 

 ments. The surveyor requires to be pro- 

 vided with a case of drawing instruments, 

 containing compasses, steel pen, parallel 

 ruler, etc.; and, also, a plotting scale, 

 graduated into chains and parts of a 

 chain. The paper on which the plotting 

 is to be made should be dampened on 

 the back and fastened down by pasting 

 the edges on a drawing-board of suitable 

 dimensions. When dry, the paper thus 

 prepared presents a smooth and uniform 

 surface; whilst the board on which it is 

 laid prevents the points of the compasses 

 from piercing deeply into it and making 

 unsighdy holes. 



