ON PLANIMETEKS. 



513 



Amsler, too, has since 1882 made planinieters with the wheel rolling 

 ou a disc. 



Soon after the construction of these instruments, Amsler published his 

 paper ' Neuere Planimeter-Constructionen ' (' Zeitschrift fiir Instrumenten- 

 kunde,' 1884), which is full of interest. He describes a planimeter in which 

 a wheel rolls on a sphere, and which is designed to measure areas on a 

 sphere or globe instead of a plane. He shows that all known planimeters 

 in which a recording wheel is employed may be considered as special cases 

 of his new instrument. Of course, if the radius of the globe is taken in- 

 finitely large, we get a plane. 



By introducing a cylinder instead of the wheel he gets an instrument 

 where there is no slipping, and he says that this planimeter is the only 

 one which deserves the name of ' precision planimeter.' This instrument 

 has never been made. In fact, Amsler considers it too complicated, and 



Fig. la. 



■\r> 



states that, although he had sold (up to 1 884) over 1 2,000 polar planimeters, 

 only a few hundred of his more complicated Moment-planimeter had 

 been made. 



Influenced, no doubt, by this paper and the criticism it contained, 

 Coradi made soon after what he calls a ' Precisions-Kugel-Planimeter.' At 

 the cost of retaining a very slight amount of slipping he simplifies Amsler's 

 mechanism considerably, and produces a simple and liandy instrument, 

 of which in four years, up to 1892, over 450 have been sold. It is made 

 in two forms. In the one, the point Q of the ' rod ' QT moves in a circle • 

 in the other, along a straight line, it being mounted on a carriage with two 

 wheels. 



I shall describe the former. A heavy disc with centre O, fig. 13, has 

 a raised circular rim B'BB" with small teeth. Round O the ' arm ' OQis 

 movable, and at Q the ' rod ' QT is jointed to it. This gives an ordinary 

 1894. LL 



