346 



Pleasant Ways in Science. 



considered — firstly, the elimination of friction, so that the least 

 possible obstacle of that kind may hinder the free motion of 

 the beam ; and, secondly, the arrangement of the point of 

 suspension above the centre of gravity, so that the beam may 

 be in stable equilibrium, to the extent of always desiring to 

 return to one and the same position, but with this tendency 

 sufficiently weak as to be counteracted by a slight force. The 

 exact distance between the centre of gravity and the point of 

 suspension must be determined by the use to which the scales 

 are to be put — the greatest delicacy and freedom of motion 

 being required for the most precious articles, and the most 

 accurate weighing. 



It seems a sudden jump from a pair of scales to a per- 

 cussion cap, but yet the transition is easy. A good pair of 

 scales are just within the conditions of stable equilibrium, and 

 a percussion cap contains a substance — fulminate of mercury — 

 which is just outside those conditions. Chemical forces are 

 capable of balancing each other as mechanical forces do. The 

 balance may be iipset with difficulty, and then the body belongs 

 to the class of stable compounds, such as all ordinary earths 

 and stones ; or it may be upsettable with facility, and then the 

 body belongs to the class of unstable compounds, of which 

 fulminate of mercury is one, and which we find is decomposed 

 by a smart blow. Some bodies of this latter kind, such as 

 chloride of nitrogen, explode violently on mere contact with 

 most other substances. 



A body may be in the state of rest called equilibrium, and 

 yet be far from actual and absolute repose. Thus a scale beam, 

 equally heated throughout its length, is in active internal 

 motion ; but as both arms are lengthened at the same time, 

 and in the same proportion, the equilibrium of gravity is not 

 disturbed. Astronomy furnishes remarkable instances of 

 groups of balanced motions producing the equilibrium of 

 systems, every part of which moves in harmony with the rest. 

 Astronomy also brings before us the conception of compensated 

 disturbances, or aberrations, an arrangement which permits 

 certain departures from the main plan, in such a manner as to 

 consist with the stability of the whole. If we descend from 

 great things to small, and pass from Nature's grand clockwoi'k 

 in the starry orbs to man's little clockwork with his chro- 

 nometers, we find analogies in the mode of operation; and 

 that form of pendulum, in which the expansion and consequent 

 lengthening of the pendulum-rod is counteracted by the 

 expansion and rise of mercury in the glass vessel which consti- 

 tutes the pendulum-bob, illustrates to some extent the nmtually 

 compensating irregularities by which the true relation between 

 suns and planets is maintained. 



