522 



REPORT — 1894. 



the pole at O. TQ is produced to D, and the extra bars OC, CD are added, 

 with hinged joints. If T describes a closed curve, D will do the same, 

 whilst Q and C are zero-points, provided that QT does not complete a 



Fig. 17. 



revolution. Let there be registering wheels on TQ and CD, and let their 

 ' rolls ' during a circuit described hj T he w and w' ; then is 



(T)=QT . tt-, (D)=QD . i«=CD . w' 



From which, on eliminating w, we get 



Hence the area (T) can also be measured by the roll of W. 



Similarly, if DC be produced to T', the area of T' will be measured either 

 by W or by W. 



If OQDC be made a parallelogram and T, O, T' are taken in a line, then 

 T and T' will describe similar figures. 



On this principle the firm Amsler-Laffon has recently constructed a 



Fig. 18. 



planimeter for measuring small areas. For convenience of construction 

 the points T and T' are made to describe figures which are only approxi- 

 mately similar, but sufficiently to make it easy to guide the tracer T so 



that T' describes a given small curve. 



