ON AMSLER S PLANIMETER. 



401 



On Amsler's Planimeter. By F. J. Bramwell, C.E. 



[A communication ordered by the General Committee to be printed in extenso.'] 



This machine for measuring the area of any figure, however irregular, by the 

 mere passage of a tracer round about its perimeter, has now been in use for 

 some years ; but, so far as the writer is aware, no easily intelligible statement 

 of its principles of action has ever been made public. 



Although no doubt the mere construction of the planimeter is now generally 

 known, it may enable the explanation which is about to be offered to be 

 more easily followed if a sketch of the actual machine, as at work upon a 

 map, be given here (see fig. 1). 



Fig. 1. 



Assume the planimeter to be anchored by its point X, and the tracer T to 

 be at some place, say A, on the circumference of the area to be ascertained ; 

 and assume the indices on the first wheel E and on the second wheel S to be 

 at zero, and that then the tracer T be carried along the perimeter of the 

 area in the direction of the arrows (with the sun), the indices will give a 

 reading up to four figures, which will represent square inches, to two places 

 of whole numbers and to two places of decimals. 



This movement of the indices is eff'ected by the wheel E, the edge of which 

 bears upon the paper, so that as the tracer T is made to go round about the 

 figure to be measured, the wheel E, from its contact with the paper, receives 

 rotary motion, and by means of the worm-pinion L and worm-wheel u, 

 communicates a diminished motion (1-lOth) to the horizoiital wheel s. 



1872. 2 E 



