106 



Electrii'itv is devdoticil in this machino by two circular plates of glass, armed with thin 

 copjiLT plates i)laced at distaiices apart, being set in motion and rotated in contrary directions, 

 tlie electricity is then picked up hy l)rass wire-brushes and jjassed in sjiarks from one electrode 

 to the other. Adjoining is another most interesting Exhibit. 



THE PLANETARIUM. 



This is a contrivance for exhibiting the relative ])ositious and motions of the jilanets and 

 their satellites. 



The Sun is represented by the large brass ball in the central position of the apj)aratus, and 

 the eight smaller globes held at constant tlistances from it by means of rods are the planets. The 

 still more minute sjiheres which are seen in tlu; vicinity of each jilanet (with the exception of the 

 two nearest to the Sun) are the satellites or moons. These are also held a,t definite jiositions from 

 the respective primary by separate systems of rods. It will be seen that the eight principal rods 

 at one end of which tlie jdanets are attached are fixed at the otlier end to a vertical shaft. This 

 shaft passes through the Sun, and is connected to clock-works below ; so that when the clock is 

 started, the shaft turns from' left to right, and causes the rods to revolve. Consequently, the 

 planets with their moons ai'e cai-ried uniforndy siround the Stm from left to right, or in a, direction 

 opposite to that in which the hands of a watch appear to move. If now we regard the left-hand 

 side as the west, then all the motions in the ajiparatus (with two exceptions referred to later) take 

 place from west to east, wliich is in accor<lance with what really occurs in the heavens. 



AVhile the planets thus revolve around the Sun, carrying the satellites on their long 

 joui'uey, the satellites circulate indejiendentlv a,r(nind their res]iective ])vima,ries. This celestial 

 jihenomenon is i-eproduced in the ap])aratus by separate gearing, wliicli gives an indejiendent 

 movement of revolution to the rods connected witli the moons, as obviously seen in the case 

 of our Earth and Moon. 



Again, ca.('h planet spins round its own axis also from left to I'ight, or from west to 

 east, and this motion is also shown by the ajijiaratus. We see, in fact, tliat whiles the Eartii 

 goes once round the Sun, it revolves 30;") times on its own axis, and in the same periotl the 

 Moon makes about thirteen revolutions aroimd it. 



The period of revolution anmnd the sun corres]ionds to the year, that of rotation on its 

 axis is the ilay. and the i'e\ohition of tlie Moon round the Earth gives the lunar month. 



The rods which connect thi- ]ilancts with tlie Sun, and those to whicdi the moons are 

 attached, may be considered in regard to their acti(»n as representing the actual forces exerted by 

 the Sun on ttie jilanets and by tlie planets on the satellites, the effect of which is to ]irevent thes(! 

 hea,\eidy bodies from flying aw;iy at a. tangent and to compel them to describe closed orbits. The 

 floor of the instrument represents a plane parallel to the eclijitic^ Here are shown the signs of the 

 zodiac, the ])oints of the compass, the months of the year, itc. : and serve to ascertain in what ])art 

 of the starry heaAens the jilanets appear at any given time of the year. 



This is the general outline of what the instrument is meant t<i imlieafe. AVe may now see 

 what llie princijial dimensions are. 



Mcrcuri/. — This is the planet nearest to the Sun, its distance from it is 36,000.000 nnh's, 

 its diameter, 3,000 miles. (iocs round the Sun in xn days. Has no satellites. The time of 

 rotation on its axis is not certain. 



Venus. — Distance from Sun, 67,0(10,000 miles: diameter. T.7oO miles, ({oes round the 

 Sun in ■-.'25 days. Its ])eriod of rotation aromid its own axis not quite certain yet. 



The Kurth. — Distance from Sun, 93,000,0t)0 miles ; diameter, 7,'JIS miles. Goes round 

 the Sun in a year ; spins round its own axis in 24 hours. 



The Monti is the only satellite of the Earth. Its distance from it is 239, ooo miles, its 

 ilianieter, 2,103 ndles ; goes rcamd the Earth in a lunar mouth, viz., 27| days. 



