LESSONS IN LAND-SURVEYING. 



LESSONS IN LAND-SURVEYING. IIL 



FIELD-BOOK AND BUHVKT OF AN BUT ATE. 



IN oar last lesson we explained to the student tho oonatrno- 

 tion and UMO of that motit valuable instrument to the land- 

 surveyor, tho theodolite. 



In previous portions of this series wo pointed oat that when 

 certain elements of a triangle are known, tho others can be 

 fnuiiil by calculation ; and it may be hore stated as a general 

 fact, and one which 

 the intelligent tu- 

 lent will see cannot 

 be otherwise by a 

 <-;ircful ri'iisi'iiiiig 

 IIJMIII till' fuliditionH 



necessary to the 



construction of a 



triangle, that, as- 



suming every tri- 



angle to consist of 



six elements 



namely, three angles 



mid three sides if 



any three of these be 



pvi'ii, tho three an- 



jrlfs aloiu- i'x.v|,t.'il, 



the others can be 



found. 



Hence tho invalu- 

 able aid rendered 



by tho theodolite to 



tho surveyor, ena- 

 bling him to ascer- 

 tain angles when 

 cannot be 



measured, and from 



them deducing the 



sides. We readily 



admit that the ac- 

 curacy obtained by 



the use of the chain 



and off-set staff is 



such, that whenever 



these instruments 



con be correctly 



employed, it is not 



necessary to call in 



the aid of the theo- 

 dolite ; but in an 



extensive survey it 



will bo impossible 



to use these simple 



means alone, and 



hence the necessity 



for employing the 



theodolite in such 



instances. 



We will endea- 

 vour to give an ex- 

 ample of our mean- 

 ing. Suppose it be 

 the survey of Scot- 

 land which wo aro 

 considering, with its 

 sea-indented coast, 

 its intervening 



lochs, mountains, and ravines, and the numerous islands dis- 

 persed around it. How would it be possible with anything 

 like correctness to measure a lino from Ben Nevis in the county 

 of Inverness to the Paps of Jura off Argyleshire ? And yet 

 these mountains are visible from each other. Suppose, however, 

 another mountain, accessible from either one or the other of 

 these, and visible from both, as, for instance, Ben Cruachan 

 in Argyleshire ; now, if the distance between the two acces- 

 sible points be accurately measured, the theodolite will enable 

 us to ascertain the correct distance between the inaccessible 

 positions. 



Let A and B in the annexed diagram represent respectively the 



129 N.E. 



positions of Ben Nevis and Jar* these two potato being 

 inaccessible to all chain measurements and c be toe position of 

 Ben Crnachan, accessible from A, and visible both from A and B. 

 Let the line A c be accurately measured. Then, 

 planting the theodolite at A, measure the angle 

 CAB, and then, placing it at c, measure the 

 angle ACS. Now the distance from A to B 



A -? y in which the ^ ABC 



in. A 



180 -mtm of angle* 



A c B and c AB ; ia 

 like manner the di*. 

 toaoe from c to B a* 



in, CAB 

 in. ABO 



BoppoM AC = 50 



mile*, ^ c A B = 

 85, and A c B = 

 75; then ^ ABO 

 = 180 (85 + 

 75) = 30 ; and 

 50X-965M* 



PLAN OF FORTESCUE MANOR FARM. 



= 141-2 mile* ; and 

 50 X 99619 

 c B - -34202 

 = 145-63 mile*. 



We shall clou 

 oar subject by giv- 

 ing the student a 

 field-book suffi- 

 ciently extensive 

 and varied to enable* 

 him to bring into 

 practice, in plotting 

 oat the property 

 surveyed, all tho 

 roles he is likely to- 

 require, except in . 

 the higher branches . 

 of our subject, such 

 as town-surveying 

 for drainage, laying 

 ont railways and 

 railway carves. 



We give some 

 general directions 

 and explanations of 

 the method we have 

 adopted, bat we 

 may observe that 

 different surveyors 

 adopt modifications 

 to suit their own 

 ideas. The points 

 to which we are 

 about to call the 

 attention of the 

 student will be all 

 found exemplified 

 in the following 

 field-book, "Field- 

 Book of the Surrey 

 of Fortescuo Manor - 

 Farm," which wfljf 

 be found in the 



next two pages. In this field-book the reader moat be careful 

 to notice that each of the four columns into which the pages are 

 divided, represents a page of the blank book with a double 

 column ruled down the centre, in which all the measurements 

 are noted. It will also be noted, as we have explained before, 

 that it is necessary to commence at the bottom of the page, and 

 proceed upwards in entering the measurements along each line, 

 ruling a double line across the page when each line is completed. 

 The large fi-jures in circles in the field-book refer to tho linos oq 

 the above plan along which the measurements are made. 



The student most refer to his tables of unoo. 



