OBJECTS.] INTRODUCTORY. 47 



of the stream is, as it were, caught and transferred to 

 the water-wheel, which consequently turns round with 

 a certain velocity. 



But this water-wheel is now a mass of matter in 

 motion, and therefore itself contains a store of energy 

 or power of doing work. If a cord with a weight 

 at the end of it were fastened to the axle of the 

 wheel, the cord would be wound upon the axle, and 

 the weight could be raised, or, in other words, so 

 much work would be done by the turning of the 

 wheel ; and we should thus have a rough measure of 

 the amount of energy which had been given up by the 

 stream to the wheel. 



The machinery of the mill is simply a set of con- 

 trivances for transferring the energy stored up in the 

 water-wheel to the place in which work has to be 

 done. In a flour-mill, for example, a series of wheels 

 carries it from the water-wheel to the grindstones, which 

 it sets in motion. 



30. The Properties of Water are Constant. 



If, whenever there is a shower, you catch some rain- 

 water, you will find that it possesses all the properties 

 which have been described. It will be found to be 

 an almost incompressible liquid, an imperial pint of 

 which weighs about a pound and a quarter. It would 

 make no difference if the rain-water were collected in 

 Africa or in New Zealand ; or if it had been obtained 

 centuries ago and kept bottled up ever since. And 

 there is every reason to believe that rain-water will 

 have exactly the same properties a hundred or a 

 thousand years hence. So far as the properties of 



