28 SCIENCE PRIMERS. [MATERIAL 



the cause of the pressure which we feel when bodies 

 which possess gravity are supported by our bodies, 

 and the cause of their movement towards the centre 

 of the earth, when they are free to move. But it is 

 exactly about the cause of these phenomena that we 

 know nothing whatever. 



A good deal of mischief is done by the inaccurate use 

 of such words as attraction and force, as if they were 

 the names of things having an existence apart from 

 natural objects, and from the series of causes and effects 

 which are open to our observation ; while they are, in 

 reality, merely the names of the unknown causes of 

 certain phenomena. And it is worth while to take 

 pains to get clear ideas on this head at the outset of 

 the study of science. 



Let us remember then that, so far as we know, it is 

 a law of nature, that any two material bodies, if they 

 are free to move, approach one another with gra- 

 dually increasing swiftness ; and that the space over 

 which each travels before the two meet, is inversely pro- 

 portional to the quantity of matter which it contains. 

 Attraction of gravitation is a name for this gene- 

 ral fact ; weight is the name for the fact in the case 

 of terrestrial bodies ; force is a name which we give 

 to the unknown cause of the fact. The fact is that 

 which it is important to know. The names are of no 

 great consequence so long as we recollect that they 

 are merely names and not things. 



20. The Weight of Water is Proportioned 

 to its Bulk. 



We must next consider, not weight in general, but 

 the weight of water. We say that a tumbler full of 



