American method of levelling for Sections. 



43 



Akt. IV. — An Exposition of the American method of levelling 

 for Sections — its superiority to the English and French 

 methods as regards actual feld practice and subsequent 

 plotting of the Section. By Martin Gardiner, Esq., (J. E., 

 Member of the Mathematical Society of London. 



[Read before the Society June 2nd, 1869] 



Foem or the Field Book. 





HEIGHTS OF 





HEIGHTS 







B.S. 



AXIS OF 



INSTRUMENT. 



F.S, 



ABOVE 

 DATUM 



STATIONS 



OBSERVATIONS. 









42-14 





B.M. on stump 20 ft. 

 W of station 1. 



3.24 



45-38 



8-36 



9-24 



10-80 





1 



2 

 3 









1336 



32 '02 





Temporary point on 

 rock. 



2-34 



34-36 



6-24 





4 









4-30 





5 



Centre of turnpike 

 road. 







8-42 





6 









11-60 





7 









12-24 



22-12 





B.M, on boulder 30 ft 

 west of station 7. 



The Field Book is divided into six columns : — 



The first column headed B,S. is for registry of back sights. 



The second is for heights of axis of instrument above datum. 



The third „ fore sights. 



The fourth ,, heights of stations and bench marks above datum. 



The fifth „ chainage of statioDS. 



The sixth ,, observatiens concerning stations, bench marks, &c. 



It may be observed that the first thing to be done in starting 

 work is to enter the height above datum of some bench-mark, say 

 42'14 in column four, and the particulars concerning it in column 

 six. Then setting up the instrument in such a position as to 

 obtain a favourable reading of the staff held on the bench-mark 

 (and, to the best of our judgment, so as to be enabled to get 

 favourable readings, when necessary, in the direction in which 

 the work is to proceed) take the reading, say 3 -24, and enter it 

 in the column of Back Sights. 



This reading added to 42 14 the height of the bench-mark, will 

 evidently give the height of the axis of the instrument in the 

 position in which we set it. 



Now, having the height of the axis of the instrument, it is 

 evident that by subtracting therefrom the readings of the staff 



