114 THE ADDRESSES, LECTURES, ETC., OF 



forwarded by Prof. Reuleaux, of Berlin. M. Tresca will bring 

 before us his interesting subject, the flow of solids. Mr. William 

 Hackney Avill 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 sur- 

 prising growth of the electric telegraph. 



MEASUREMENT. Regarding the question of measurement, this 

 constitutes perhaps the largest and most varied subject in connec- 

 tion 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 dis- 

 coveries 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 dis- 

 covery was made. It was by a long train of mathematical calcu- 

 lation, 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 moon towards the earth. Then 

 first, we may suppose, came to him the idea of the universality oj 

 gravitation ; but when he attempted to compare the magnitude of 

 the force of the moon with the magnitude of the force of gravita- 

 tion of a heavy body of equal mass at the earth's surface, he did 

 not find the agreement which the law he was discovering required. 

 Not for years after would he publish his discovery as made. It is 



