III. AREA. 61 



317. Polarplanimeter. A* Ott and G. Coradi, Kcmpten. 



By means of the polarplanimeter the superficial contents of any kind of 

 figures drawn on paper, no matter what their outline may be, can be ascer- 

 tained by mere tracing more exactly and quickly than by any other method. 



The inventors of this instrument are respectively J. Amsler, in Schaffhausen, 

 and Ch. Harke, in Vienna. Ott and Coradi's construction is a combination 

 of both, embracing the excellences of each. It differs from Amsler's instru- 

 ment by the pole (axis) of the instrument not being formed by an inserted 

 point of a needle, but by a steel ball embedded in a metal cylinder, thus giving 

 it a firmer position ; and, moreover, by the axis of the roller being lodged in 

 a horizontal frame, and the dividing circle of the roller (cylinder) as well as 

 the indicating wheel being free at the top, thereby affording much easier and 

 more accurate reading than Amsler's instrument. This arrangement has the 

 advantage that for simple calculation the zero point of the drum can be placed 

 exactly on the zero point of the nonius, when the tracing pencil is at the 

 commencement of the figure. The weight can be separated from the instru- 

 ment, by withdrawing the bolt, and placed in the case by itself. The runner 

 carrying the axis of the polar arm can be moved along the whole length of the 

 quadrangular bar, by which means at every agreeable longitudinal scale a 

 round number can be obtained for the value of the nonius unit (for example, 

 scale 1 500 nonius unit, 2 square meters, or scale 1 1440 nonius unit, 5 square 

 fathoms). The tracing bar is divided into ^ mm., and the runner sliding on 

 the same carries on one side a nonius, on the other an index. For adjustment 

 with the index, the most usual or specially desired longitudinal scales are 

 marked with lines on the bar ; by means of the nonius and the divisions on the 

 bar, proportions of measure not previously given can be easily inserted and 

 noted down ; in the same manner, in the case of plans which have been drawn 

 on shrivelled paper, the area can be retained in its actual size by a corre- 

 sponding movement of the runner, and the position of the nonius noted down 

 for a certain amount of shrivelling. 



318. Planimeter, divided on a glass plate, in a case. 



F. W. Breithaupt, CasseL 



The planhneter consists of a net marked on a glass plate for a certain 

 relation of the meter measure. 



319. Wetli's Planimeter. 



Physiological Institute of the University of Halle (Prof. 

 Bernstein, Director). 



The planimeter is fitted together by placing the six-toothed movement into 

 the centre of the divided disc, whilst the central point of the small glass disc 

 moves at the other end in the screw of the hoop encircling the divided disc. 

 Next, the sledge with 4he large glass disc is placed on the three-railed track 

 in such a manner that the horizontal glass disc comes underneath the smaller 

 vertical one ; the latter is then, by means of the screw which is fixed on the 

 hoop, regulated in such a manner that it is easily carried along by the hori- 

 zontal disc by friction. 



The pointer moving with the same on the same axis on the dividing disc 

 indicates by a' pencil, easily observable by watching the instrument, the figure 

 according to its superficial contents in quadratic millimeters. The toothed 

 little wheel contrives a reading of every 1,000 quadratic millimeters of the 

 traced outline of the surface. 



