PRESENT FUNDAMENTAL CONCEPTIONS OF PIIYSICS. 



493 



conclusions to be drawn from them. The short space of time allotted 

 us obliges us to use economy, and therefore we limit ourselves to the 

 indispensable in this direction. First, however, it will be necessary to 

 define what (according to Poncelet and Coriolis) is termed " work " in 

 scientific language, how it is measured and calculated. Whatever effect 

 of any power we may subject to analysis, we shall always arrive at the 

 result that the " work" performed by a force consists in the overcoming 

 of a resistance within a given distance. For instance, if a force has to 

 overcome a ten times greater resistance over the same space, its work 

 will be ten times greater, and so the labor of a force is three times greater 

 if it overcomes a uniform resistance over a three times greater space. 

 The amount 01 work by a force, therefore, increases in proportion to the 

 resistance which is overcome and to the length of way over which the 

 resistance has been overcome. If the work of forces is to be expressed by 

 numbers, it is necessary to name a work unit. The work required to lift 

 1 leg. through 1 meter is called a kilogram-meter or meter-kilogram (kgm), 

 ami is regarded as the work-unit. If then there is a resistance of 10 

 Tegs, to be overcome through the space of one meter, the work is 10 

 Jcgms. ; but if this resistance is to be overcome over three meters, the 

 work will be three times 10 kilogram-meters, that is, 30 legms. We see 

 from this that the amount of any work is calculated by multiplying the 

 distance by the force. We are now enabled to include within our ex- 

 aminations the most easily comprehensible of Joule's experiments. A 

 perpendicular axis of ro- 

 tation, A A, (see figure,) 

 ^provided with brass pad- 

 dles F F, — having their 

 planes F in an upright 

 position, was placed in a 

 weighed quantity of water, 

 W ; this shaft Joule caused 

 to rotate by means of a 

 weight, P, drawing a cord, 

 S S, wound on the drum, 

 A ; the circulating current 

 of the liquid W being pre- 

 vented by rigidly fixed 

 brass frames, m m. The 

 friction of the water W 

 against the planes F of the 

 paddles, the frames m m and its own parts caused a rise of temperature 

 in the liquid, which was measured by means of excellent and very sen- 

 sitive thermometers, T T. The multiplication of the amount of the 

 fallen weight P by the distance it had travelled n v gave the measure 

 of the mechanical labor used for the friction. If, then, the friction of 

 all the movable parts outside the water-tank of the apparatus, and all 



