SPEEDY ELEVATOR. 523 



made of theie can under flender forms bear weights many 

 times greater then their own. Thefe qualities are in their 

 blended effects of different value in this machinery : the 

 pillars are the moft prefled, but they caufe no weight to 

 the power, and therefore their bulk is the lefs detriment. 

 B B being the heavieft laden piers are the moft folid, but 

 they have only a fimple moment : C C bearing lefs are 

 lighter, but their moment is double : D has a treble moment, 

 but the lighteft burden, and thus the leaft w^eight of its 

 own. Thefe continually growing increments of folidity are 

 necelfary confequences of the conftant double bearings ; 

 but ought to be fmall in comparifon to the preffures thus 

 produced, which become very great, when the load to 

 be lifted, and the elevation are confiderable. Lightnefs 

 is then moft beneficial in the upper ranks, and firmnefs 

 in the lower, as thefe mujl lift but thofe be lifted many 

 times : accordingly different forts of wood may be chofen 

 by their degrees of lightnefs and firmnefs ; they being 

 otherwife proper, efpecially for clofe and fmooth folding. 



On account of the grooves and tongues the pieces can- 

 not have thofe regular forms that give the greateft folidity; 

 nor can the pulleys be placed exadtly over the line of cen- 

 tral ftrength. In pracStife thefe defe<3:s muft not exceed 

 necefl'ary limits. Moreover, when the preffures and ftrains 

 on the feveral parts of the pieces are eftimated, hollows 

 may be contrived in places that can bear it — Thefe niceties 

 cannot be marked in a model. 



When the load, the elevation, and quality of the^'ood 

 are given, the lighteft feries of piers is found by comput- 

 iug the refults from different numbers.* A greater number 

 muft effe£l a greater proportion of the whole elevation than 

 a fmaller, becaufe the pillars, by becoming ftiorter, con- 

 tribute lefs ; this addition is a new expenfe of power. The 

 3 Y 2 weight 



* In this the pairs are confidered as one. 



