ON GRAPHIC METHODS IN MECHANICAL SCIENCE. 401 



Another example of plotting, whicli the writer believes to be novel, is 

 the fall of polarisation of batteries used for telephones, &c., and the 

 gradual rise of electro-motive force afterwards, as shown in fig. 30. 



This has in one or two cases led to the discovery of the cause of 

 failure in telephonic arrangements to be due to the batteries, and not to 

 the telephone, as was at first supposed. 



Before leaving the subject of plotting diagrams, it would be as well 

 to remark that the examples which have been given must necessarily be 

 but a small proportion of the cases which might be cited. Thus, in many 

 treatises in which graphic representations are given the action of forces, 

 such as diagrams of bending moment, shearing force, moment of inertia, 

 velocity, efficiency, &c., which may be calculated graphically, is fre- 

 quently calculated by numerical methods, and afterwards merely plotted, 

 so as to present the results of calculation in a graphic form. 



2. Instruments for Drawing Known Curves, 



The use of the ruler and compass for drawing known curves is 

 postulated in Euclid, and they still retain the place of honour, as by far 

 the most important instruments of the kind in graphic methods. 



It is often required to draw a straight line, without using a ruler, with 

 indicator diagrams, and for other work, and then some form of straight- 

 line mechanism must be used. A valuable amount of information on the 

 subject is to be found in the treatises of Professor Goodeve,' Professor 

 MacLay,^ Mr. Kempe,^ and Professor Kennedy."* 



In an article by the late Professor H. J. S. Smith •'' in connection with 

 the Collection of Scientific Apparatus of 1876, the following improve- 

 ments in recent years in compasses are noted : — ■ 



1. The arrangements adapted for very fine work, and known as hair- 

 compasses, needle-point compasses, and spring-dividers. 



2. The proportional, or reducing compasses, by which we are enabled 

 to reduce or augment in any given ratio the distances which we transfer 

 from one drawing to another. 



3. The triangular compasses, by which the position of three points 

 forming a triangle can be transferred from one drawing to another, and 

 which thus serve as an instrument for transferring angles. 



4. The beam-compasses, consisting of a beam or bar along which the 

 two points of the instrument may be moved backwards and forwards, the 

 distance between them admitting of adjustment with great precision by 

 means of a micrometer screw. 



"With regard to the other instruments for drawing curves he remarks: — 

 ' It is by no means an easy matter to invent a good method of 

 tracing a curve. Even when the theory of a curve is pretty well 

 known, that theory may fail to suggest any mode of describing it 

 mechanically ; and not every mode which theory suggests can be made 

 to work accurately in practice.' He afterwards mentions elliptic trammels 



' Elements of Mechanism, by Professor Goodeve. Longmans, Green, &. Co. 

 ^ Meclianical Drawing, Part I., 'Loci,' by Professor A. MacLay, B.Sc. 

 ' How to Bram a Straight Line, by A. B. Kempe {Nature Series). 



♦ Mecluinics and Machinery, by Professor A. B. \V. Kennedy. Macmillan k. Co. 



* Handbook of the Special Loan Collection of Seientijic Apparatus. Chapman k. 

 HaU. 



1892. D D 



