142 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



power required being in proportion to. the area of the plates. 

 About 2^ to 3 inches in diameter is the usual size. 



Price, $1.50. 



Bohnenberger' s Machine. This apparatus (See Fig. 146, 

 below) consists of three movable rings, A B C, mounted on 

 a stout base. The two inner rings are mounted on pivots ; 

 those on the smallest ring at right angles to the middle one ; 

 in the smallest ring is supported a metal ball, having a 

 roller on one of its pivots ; around the roller a string may 

 be wound, and when pulled off a rapid rotary motion may 

 be given to the ball. This motion may be given with its 

 axis in any position required, and when communicated, the 

 ring supporting the ball will resist considerable effort to 

 alter its position, and whatever way the instrument may be 

 turned, its axis will continue to maintain the position it had 

 when set in motion, illustrating the inertia, or that property 

 of matter which resists any change of state, whether of rest 

 or motion. Price, $5.00. 



Fig. 146. 



Fig. 147. 



Instrument for illustrating Impenetrability . (Fig. 147, 

 as above.) By impenetrability is meant the fact, that two 

 bodies cannot occupy the same space at the same time. 

 The instrument to illustrate this principle consists of a glass 

 jar, A, to which is fitted, air-tight, a large cork, through 

 which a funnel, B, with small neck, and bent glass tube, C, 

 are inserted ; one end of the latter is to be immersed in a 

 vessel of water, D. If, now, water is poured into the fun- 

 nel and enters the jar, it will be found that the same 



