98 THE AKCH. 



say, 7-^^ foet; it may be conceived that if two of tlieso stones 

 be placed against the abutments and the third in the 

 middle between them, as shown in the sketch, it might be 

 possible so to place them that they would balance each 

 other and remain as a rude self-supporting arch, notwith- 

 standing the so placing of them by hand would be a work 

 of the greatest difficulty. 



Now, though we may be unable to do this by hand, yet 

 Nature will solve the problem for us at once in this way : 

 Let us only imagine that the stones are strong magnets, 

 and that the three stones and the abutments are turned 

 upside down, so that the three stones shall be in suspension 

 from the abutments, perfectly free to move at the joints, 

 but held closely together by our supposed magnetic attrac- 

 tion ; then the three stones woiild naturally fall at once 

 into their true positions, and if we could ordy replace them 

 in these exact relative positions when again turned up, we 

 should have a perfectly balanced structure. 



This principle of inversion and suspension is true of all 

 balanced arch structures, whether they are of the simplest 

 form, as in this case, or in any more complicated form, and 

 whether they are arches of construction or inverted arches 

 of suspension ; both are dependent on the same vertical 

 force of gravitation combined with horizontal thrusts. All 

 that is necessary is only that the structure should bo 

 balanced. The difference between the two forms being, that 

 the arch has to be constructed in truly balanced form, while 

 the inverted arch falls of itself into its perfectly balanced 

 position. The one is equipoised by art, the other by 

 Nature. 



If, for example, we take a string of beads, and hook up 

 the two ends of the string, we know that the beads will 

 range themselves immediately into the form of an inverted 



