ON GRAPHIC METHODS IN MECHANICAL SCIENCE. 



389 



readily deterraiued. The ' Pocket-Book ' of Trautwine on earthwork con- 

 tains a large number of cases in which the results of elaborate tables are 

 plotted, and can be used much more readily than the tables themselves. 



Many other examples might be cited, such, for instance, as a curve 

 given by M. Marey,' in which are plotted various data concerning suspen- 

 sion bridges of a certain type, the weights respectively of iron and other 

 metals, and wood, as well as the cost, being given for every span within 

 a certain limit. 



In fact, there is no limit to the possible number of graphical applica- 

 tions, since they can be employed whenever the result is required of two 

 quantities which vary simultaneously. It must be remarked, however, 

 that the result given by their use cannot be regarded as very accurate, 

 though probably they admit of as close a degree of accuracy as it is 

 usually possible for an engineer to attain in practical work, and certainly 

 have the unquestionable merit of preventing any great error which may 

 occur as the result of calculation, which therefore they may well be used 

 to check. 



(3) The last, and possibly the most important, object of plotting results 

 is the regular work of the engineer or naval architect, in which he sets 

 down the results of a few calculations in order to be able to determine 

 any others which he may require in the same series. This, as graphi- 

 cally treated, becomes a very simple process, and amounts to merely draw- 

 ing a curve through the points which he has found and plotted, and 

 interpolating all the values that he requires. This, if performed mathe- 

 matically, would require difficult and often impossible calculations, and 

 it is just for such purposes as this that the graphical method is indis- 

 pensable to, and will be invariably adopted by, practical men. 



Mr. Henry H. West, of Liverpool, M.Inst.C.E., Member of the Coun- 

 cil of the Institution of Naval Architects, has kindly supplied the writer 

 with the following remarks, which admirably illustrate this point in 

 connection with naval architecture : — 



' The principal use which naval architects make of graphic methods is 

 to record in the form of curves the results of calculations made for certain 

 given conditions, and from these to arrive at similar results for other 



Fig. 13. — Curve of Displacement. 



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conditions by interpolation of new points in the curves corresponding 

 with the required new conditions. Beyond this, graphic methods are 

 little, if at all, used as a means of calculation. 



' La Methode graphique, par E. J. Marey. G. Masson, Paris. 



