ON GKAPiriC METHODS IN MECHANICAL SCIENCE. 419 



speed-recorders, hydrometers, meters of all kinds, pressure-gauges, 

 pyrometers, tide-gauges, thermometers, ammeters, volt-meters, galvano- 

 meters, and chronometers. This is a colerably comprehensive list, and 

 a large number are also referred to in the accompanying appendix to 

 this report ; yet it is impossible to do full justice to the subject. 

 Various instruments by probably the two most important makers, Messrs. 

 Richard Freres, the French firm, and Mr. Bailey, of Salford, have been 

 referred to, as well as those of others, but a very large number of such 

 instruments have been designed and made of which the writer is obviously 

 unable to ascertain any details or give any notice. Tliere are also 

 numerous instruments made by the above makers of a compound nature, 

 recording several operations at once, which scarcely come under any 

 classification, such as instruments for exhibiting at one view time, pres- 

 sure, speed, and temperature connected with an engine, instruments for 

 giving a weekly diagram for speed, and pressure instruments for re- 

 cording the number of checks and the time given in by workmen. 



III. Solution of Problems. 



The solution of problems by graphical methods, that is to say, by 

 geometrical construction, has two aspects : — 



1. The use of known curves or simple plotted figures for problems, 

 which are well understood, and have become part of the subject-matter 

 of all engineering pocket-books. The drawing of such curves has already 

 been dealt with, and the increasing employment of 'graphic' tables 

 mentioned. Matters which a few years ago were questions of theory 

 only have now been placed by the light of experience in the catalogue of 

 cut-and-dry facts which can be plotted graphically. Take, for example, 

 the subject of girders and rolled joists. A few years ago each girder 

 was a subject of separate calculation by the engineer designing a struc- 

 ture. To-day, makers of such girders send out lists giving the sizes of 

 girders and all their proportions (including price) for the various condi- 

 tions of loading found in practice, and such lists can be, and often are, 

 put in a graphical form,^ and the engineer or architect, if he chooses to 

 trust such a table, has only to refer to it and select a suitable girder for 

 his work. 



Again, take the case of marine steam-engines and boilers. Here 

 many of the Board of Trade rules have apparently reached a final form 

 for cranks and crank-shafts, boiler-plates, stays, tubes, safety-valves, &c., 

 and might be thrown into graphical form for practical use. 



Now the use of plotted tables is really an expeditious mode of graphi- 

 cal calculation, because even the interpolation of a value, which is gene- 

 rally required, and the use of a scale lor measurement, is practically a 

 graphical process, and is clearly to be distinguished from the construc- 

 tion and drawing of such a table from given numerical data. For this 

 reason some remarks are made upon the use of such tables in this divi- 

 sion of the subject; and for the same reason, under the same heading, the 

 subject of slide rules is also considered. 



2. But in mechanical science, whatever may be the results hitherto 

 attained, problems are always arising which require solution, and even 

 in the face of achievements such as the Forth Bridge, the Eiffel Tower. 



o 



A number of examples have been already given («ee p. 388). 



E p 2 



