28"* SCIENTIFIC RECREATIONS. 



were used by him for improvising retorts, bottles and vases for forming 

 precipitates, and carrying on many important operations. 



Scheele also operated in like manner ; he knew how to make great 

 discoveries with the humblest appliances and most slender resources. One 

 cannot too earnestly endeavour to imitate such leaders, both in teaching 

 others and instructing oneself. 



The laws relating to the weight of bodies, the centre of gravity, and 

 stable or unstable equilibrium, may be easily taught and demonstrated by 

 means of a number of very familiar objects. By putting into the hands 

 of a child a box of soldiers cut in elder-wood, the end of each fixed into 

 half a bullet, we provide him with the means of making some easy 

 experiments on the centre of gravity. According to some authorities 



Fig. 21. Experiment on " centre of gravity." 



on equilibrium, it is not impossible, with a little patience and delicacy of 

 manipulation, to keep an egg balanced on one of its ends. This 

 experiment should be performed on a perfectly horizontal surface, a marble 

 chimney-piece, for example. If one can succeed in keeping the egg up, it 

 is, according to the most elementary principles of physics, because the 

 vertical line of the centre of gravity passes through the point of contact 

 between the end of the egg and the surface on which it rests. 



Fig. 21 reproduces a curious experiment in equilibrium, which i& 

 performed with great facility. Two forks are stuck into a cork, and the 

 cork is placed on the brim of the neck of a bottle, The forks and the cork 

 form a whole, of which the centre of gravity is fixed over the point of 

 support. We can bend the bottle, empty it even, if it contains fluid, 

 without the little construction over its mouth being in the least disturbed 



