Full text: Vitruvius: The architecture of Marcus Vitruvius Pollio, in ten books

310 
the mast, which is as it were the centre, but at a distance 
therefrom, they are acted on by the wind with greater 
force. For as, if the fulcrum be placed under the middle 
of a lever, it is but with difficulty that the weight is 
moved, and that only when the power is applied at the 
extremity of the lever, so when the sails are no higher 
than the middle of the mast, they have less effect on the 
motion of the vessel : when, however, raiséd to the top 
of the mast, the impulse they receive from an equal wind 
higher up, causes a quicker motion in the ship. For the 
same reason the oars, which are made fast with rope to the 
thowls, when plunged into the water and drawn back by 
the hand, impel the vessel with great force, and cause the 
prow thereof to cleave the waves, if their blades are at a 
considerable distance from the centre, which is the thowl. 
Also, when loads of great weight are carried by porters 
in gangs of four or six, the levers are so adjusted in the 
middle that each porter may be loaded with a proper pro- 
portion of the burden. The middle parts of the levers 
for four persons over which the tackle passes, are pro¬ 
vided with pins to prevent it sliding out of its place, for 
if it shift from the centre, the weight will press more on 
the shoulders of him to whom it is nearest, just as in the 
steelyard the weight is shifted towards the end of the 
beam. Thus also oxen have an equal draft when the piece 
which suspends the pole hangs exactly from the middle 
of the yoke. But when oxen are not equally strong, 
the method of apportioning to each his due labour is by 
shifting the suspending piece so that one side of the yoke 
shall be longer than the other, and thus relieve the weak- 
er animal. It is the same in the porters' levers as in 
yokes, when the suspending tackle is not in the centre,
	        
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