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A Key to Civil Architecture : Or,

round by an Helical revolution about the body ofa cylinder; receiving its motion not from a stroke,but a leaver or handle at one end of it; and is usu-ally distinguished by the names of male and female.The male is the fore-mentioned screw ; the femaleis the nut which receives it. The quality of thisfaculty far exceeds any of the rest, for that use towhich it is generally applied ; as, in printing, ex-tracting and squeezing out the juice of fruits, &c.and in the execution of this instrument the strengthof one man will be more forcible than the weightof a whole mountain. It is also used for lifting andraising great weights, and is much more practica-ble than any instrument made of wheels, pulleys,&c.

The great advantage of this faculty above therest doth consist chiefly in this; the other instru-ments do require so much strength for the support-ing of the weight to be moved as may be equal toit, beside that other fuper-added power whereby it isout-weighed and moved ; so that in the operationby these a man does always spend himself in a con-tinued labour. Thus for example :

Any weight that is lifted up by awheel or pulley,will of itself recline, if there be not an equal powerto sustain it: but in the composure of the screw,this inconvenience is perfectly remedied ; for somuch as is communicated unto this faculty from thepower that is applied to it, is still retained by thevery frame and nature of the instrument itself; sincethe motion cannot return, but by the handle of theleaver which effected it; so that all the strength ofthe power may be employed in the motion of theweight, and none spent in sustaining of it. Theprincipal defect of the screw is, that in a strort spaceit will be worked to its full length : and then it can-not