206 SECTION MECHANICS. 



addressing you on the subject of Prime Movers, and Professor 

 Kennedy upon the kinematic apparatus forwarded by Professor 

 Reuleaux, of Berlin. M. Tresca will bring before us his interesting 

 subject, the flow of solids. Mr. William Hackney will address you 

 upon the application of heat to furnaces, for which he is well qualified 

 both by his theoretical and practical knowledge. Mr. R. S. Culley, 

 Chief Engineer of the Postal Telegraphs, will refer you to a most 

 complete and interesting historical collection of instruments, revealing 

 the rapid and surprising growth of the electric telegraph. 



Measurement. Regarding the question of measurement, this con- 

 stitutes perhaps the largest and most varied subject in connection 

 with the present Loan Exhibition. In mechanical science, accurate 

 measurement is of such obvious importance, that no argument is 

 needed to recommend the subject to your careful consideration. But it is 

 not perhaps so generally admitted, that accurate measurement occupies 

 a very important position with regard to science itself, and that many 

 of the most brilliant discoveries may be traced back to the mechanical 

 art of measuring. In support of this view I may here quote some 

 pregnant remarks made by Sir William Thomson in his inaugural 

 address, delivered in 1871 to the members of the British Association, 

 in which he says, " Accurate and minute measurement seems to the 

 non-scientific imagination, a less lofty and dignified work than looking 

 for something new. But nearly all the grandest discoveries of science 

 have been but the rewards of accurate measurement and patient long- 

 continued labour in the minute sifting of numerical results. The 

 popular idea of Newton's grand discovery is that the theory of 

 gravitation flashed upon his mind, and so the discovery was made. 

 It was by a long train of mathemetical calculation, founded on results 

 accumulated through prodigious toil of practical astronomers that 

 Newton first demonstrated the forces urging the planets towards the 

 sun, determined the magnitude of those forces, and discovered that a 

 force following the same law of variation with distance, urges the nx)on 

 towards the earth. Then first, we may suppose, came to him the idea 

 of the universality of gravitation j but when he attempted to compare 

 the magnitude of the force on the moon with the magnitude of the 

 force of gravitation of a heavy body of equal mass at the earth's sur- 

 face, he did not find the agreement which the law he was discovering 



