APPLIED MECHANICS. 193 



things which have combined to give us the machinery of the pre- 

 sent day. Thus, a planing machine is an instrument, as its very 

 name implies, for multiplying plane surfaces, and it does this 

 work by transferring to the piece of metal operated on, the truth 

 of guiding plane surfaces formed first of all within itself. The 

 slide-rest of a lathe is built up of plane surfaces sliding upon 

 each other. The bed of a lathe must be a plane, or it would be 

 inadequate for the direction of the slide-rest. The lathe and the 

 planing machine are continually producing and correcting surfaces 

 required for the construction of other machines, and it is, therefore, 

 most necessary that they should themselves be perfect models. 

 Indeed, if we extend our survey beyond the workshop, it will be 

 found impossible to separate our successful manufacturing skill from 

 that primary element on which it is founded, viz., the mechanical 

 power of producing true geometrical forms. 



But recurring to the subject of the steam-engine, it is hoped 

 that many objects for the illustration of its working parts, and 

 many diagrams to aid the teacher, will now be brought together. 

 The School of Mines supplies a valuable series, while both 

 Mr. Anderson's and Mr. Shelley's diagrams present much that 

 cannot fail to be useful. We see the first conception of the 

 compound cylinder engine by Hornblower, and can pursue the 

 theory of the expansive working of steam as well as its prac- 

 tice. The application of this principle to marine engineering 

 is a matter of the highest interest, and everything that shows 

 better the mode of carrying it out must be extremely valu- 

 able. In the present notice the writer has contended for the 

 greater extension of mechanical teaching by close and exact 

 reasoning upon observed facts; and it may here be well to 

 remark that the development of the mechanical theory of heat 

 has opened out a new region for exploration. In 1872 the annual 

 production of coal in Great Britain was estimated at 120 millions 

 of tons. One pound of ordinary coal develops in its burning a. 

 number of units of force, which represent Ib. of coal for eacli 



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