188 The Foundations of Physical Science. 



A body that does not move "because the forces acting upon it 

 are balanced, is not inert in any proper sense of the word. If 

 it be a ball resting on the table, it tends towards the earth's 

 centre by an active gravitation thereto, and it does not go 

 through the table, simply because the cohesion of the wood is 

 greater than the force by which it is pressed downwards. A 

 piece of thin paper held on stretch will support a billiard ball, 

 but a pound weight would go through it. If a body is still, it 

 is so because the forces that act upon it balance each other, 

 and it will move if a fresh quantity of force be added, by which 

 it is overbalanced in any direction. In teaching mechanics, it is 

 advisable that this truth should be borne in mind, and that the 

 pupil should know that the word inertia by no means expresses 

 the actual state of the case. 



The capacity to be of service in the concerns of practical 

 life depends a good deal upon an acquaintance with the ele- 

 ments of physics, and without that knowledge it is impossible 

 to make sufficient advance in any other science to afford either 

 profit or delight. And yet hundreds of schools still exist, at 

 which boys and girls may pass seven or fourteen years without 

 knowing the difference between a pulley and a screw ! For 

 private families the means of pleasurable instruction are sup- 

 plied by Dr. Arnott's book. It should be read a chapter at a 

 time, and the various experiments performed with articles that 

 exist in every house. Pieces of stick will make levers ; every- 

 body can get a common carpenter's screw ; a cotton reel is a 

 pulley ; a teapot teaches hydrostatics, because the small column 

 in its spout is able to balance the big column in the vessel 

 itself; it will also teach some hydraulics, because, with a given 

 inclination, it will discharge its contents quicker when full, than 

 when, from being partly empty, the 'level of the source of 

 supply is not so much above the point of exit as in the former 

 case, and consequently the pressure is less. The habit of 

 understanding the scientific principles that operate in daily 

 concerns is an invaluable one. It is astonishing that men have 

 lived so long in the world, ;uid that the most civilized nations 

 are only beginning to Unci out that it is desirable to know 

 something about it. Without some knowledge of physics and 

 chemistry, without microscopes and telescopes, the mind is half 

 starved, because so Little of nature's operations is understood; 

 and there remains a, great gulf of* separation between the in- 

 structed few and the uninst ruoted man}'. Far better would it 

 be — and happily not now difficult or costly — for the average 

 cull iv;it ion of youth to lie carried at least as far as the rudiments 

 of posit ive knowledge in the departments we have specified, and 

 then tin- capacities of society for utility and enjoyment would 

 be a million-fold increased. 



