512 REPORT— 1894. 



one or the other side of it. Wilski seems to attribute it to an obliquity 

 in the joint at Q. Coradi's experiences during more than ten years also 

 reveal these maxima and minima. But he finds them equally in all plani- 

 meters where a wheel is used, and draws the conclusion that the error 

 causing them must lie in the wheel and in the resistance to its turning. 

 The slipping he does not mention, but there seems no doubt that in it lies 

 the true dynamical cause. 



Fifth error : The axis of the wheel does not Ue in the rod QT, but at 

 a small distance sideways from it. This introduces a small amount of 

 extra sUpping. 



The errors discussed so far depend on the construction, the next on the 

 skill of the manipulator. 



Sixth error : Due to wrong guiding of the tracer. 



As the tracer has to be guided by hand round the curve, there will 

 always be slight deviations. Of the error hereby introduced not much can 

 be said in general. It depends on the skill and care of the manipulator. 

 The latter should always look as much as possible in the direction in 

 which the tracer moves along its path, so that any deviation may at once 

 be detected. These deviations will, in general, be partly positive, partly 

 negative, so that the errors introduced will greatly cancel each other. 



To separate these errors from those due to the instrument it is neces- 

 sary to guide the tracer mechanically round a known area. For this pur- 

 pose Coradi uses, and sells with his planimeters, a ' Control-lineal,' con- 

 sisting of a strip of metal with a line mai'ked along the middle of its 

 length. On this line a series of little conical holes are made to receive 

 the tracer. It can turn round a needle-point at one end, which is pressed 

 into the drawing-board. But to make good tests with it requires some 

 skill. 



Elaborate tests of A'arious planimeters have been made in this manner 

 by Prof. Lorber, of Leoben (' Zeitschrift fiir Vermessungswesen,' 1883 and 

 1888). 



Rules for the accurate use of planimeters and for testing them are 

 also given by Coradi in a pamphlet ' Praktische Anleitung zum Gebrauch, 

 etc., des einfachen Polar-Planimeters,' 2te Aufl., Ziirich, 1888. 



HoHMANN- Coradi and Lang-Coradi, 



Amsler's polar planimeter remained practically unaltered till the 

 Bavarian engineer F. Hohmann constructed his ' precision planimeter ' in 

 1876. In 1880 he communicated his ideas to Coradi in Zurich, who has 

 since constructed it in a variety of different forms. 



The Hohmann-Coradi precision planimeter is of Type III. But the 

 recording wheel rolls on a disc with finely prepared surface. This disc 

 itself is turned by the motion of the point Q of the generating line or ' rod ' 

 QT. The wheel rests with light pressure on the disc. The friction to be 

 overcome by the slipping of the wheel is thus greatly diminished. 



Of the different forms used by Coradi, I mention only the ' rolling 

 planimeter.' The point Q is guided along a straight line by the aid of a 

 carriage which rests on two wheels with milled edges. The motion of the 

 instrument in this direction is thus unlimited. Such wheels without rails 

 had been used already by Sang and by J. Stadler (Dyck's ' Catalogue ') for 

 a planimeter of Type III. 



