﻿Prof. Perry on Harmonic Analysers. 125 



J, J, ... of i0x, be made to roll on flat rails lying on the paper, 

 the values of c 2 , « 2 , c 3 , a 3 , . . . are obtained in succession, one 

 pair of coefficients for each tracing of the curve. 



This instrument has the advantage over any Harmonic 

 Analyser previously designed that it gives directly the quan- 

 tities — amplitude and epoch — of each simple harmonic term 

 which are required ; all other instruments, as far as I am 

 aware, giving the coefficients A n and B n , from which C n and 

 a are calculated. 



n 



It is remarkable that no adjustments have to be made 

 before using the instrument, the initial position of the disk d 3 

 having no influence on the curve Opp'R described on it. 

 There is no part of the instrument which demands excessive 

 accuracy of construction. The accuracy and delicacy of the 

 instrument depends on the accuracy with which the line OR 

 and angle Y'OR can be measured, and will be quite as great 

 as that with which the original curve fig. 1 is drawn. 



In some cases there will be a danger that the disk d 3 may 

 not be large enough to contain the complete curve Opp'R 

 (fig. 2). If the rolling wheel W is about to roll off the 

 disk, a mark should be made with the needle n, and keeping 

 the tracer P in the same position, the disk d 3 should be moved 

 by hand into any other convenient position relative to disk d u 

 a new mark made with the needle, and the movement of the 

 tracer P may then be proceeded with. The final line OR 

 can then be easily built up from its separate parts. 



Since writing the above I have designed an inversion of 

 the mechanism described above giving a simple compact 

 instrument, which I may have the pleasure of describing 

 later on. 



X. Remarks on Prof. Henrici's Paper made by Prof. 

 Perry, F.R.S., in which he describes a Simple Machine 

 which may be used to develop any Arbitrary Function in 

 Series of Functions of any Normal Forms *". 



I CONGRATULATE Prof. Henrici, first upon his success 

 in these Analysers, with which I shall presently form a 

 practical acquaintance when the latest of them yet constructed 

 reaches me from Zurich, second on the admirably clear way 

 in which he described them to us. 



I have had no experience with the hatchet, that simplest 

 of all planimeters ; but with regard to the Robertson-Hyne 



* Communicated by the Physical Society : read April 13, 1894. 



