MILLS. 69 
slowly and requires much space. <A square frame, }, is mounted on the 
truck a, and supports the posts cc, which are united by ties and braces to 
form a square tower, the stability of which is secured by the stays na when 
mounted for use. In front and in the rear is a ladder, a, which moves on 
a hinge at top, and when in use is braced out by the strut a’. In the 
interior of this tower are two more of the same kind, sliding out like the 
tubes of a spy-glass, the second within the first, the third within the second. 
The third story has on top a platform provided with a railing, from which 
communication with the building is established by means of planks or 
ladders thrown across. The several stories are raised by means of the 
machinery seen at the bottom, which winds the ropes f and ¢ on a drum. 
IV. MILLIS. 
Before entering upon the subject of Mirzs, it may not be out of place to 
say a few words upon the power by which they may be set in motion, and the 
improvements made in modern times in this branch of industry. 
Whatever power may be made use of, it should be so arranged as to pro- 
duce a rotary motion. Man-power, horse-power, steam, wind, and water, 
may be used, or in fact any agent capable of being employed to drive 
machinery. The simplest mode of driving machinery is by horse-power. 
The horse in this case is harnessed to a long horizontal lever, mortised into 
a vertical shaft, and is forced to travel round in a circular path, and thus 
the shaft is made to revolve and to give off the power to the machinery to 
be driven. There are @ great variety of methods of making use of the 
power of horses for driving machinery, which are too special to be discussed 
in this place. 
Another common motive power for mills is wind. This power is 
obtained by the pressure exerted by the wind upon the inclined arms or 
sails of the wind-wheel, and is thence communicated to the mill. In some 
eases the whole mill is made to revolve, in order to bring the wheel in a 
proper position to be acted upon by the wind; in others, as in the case of 
the Dutch mills, only the upper portions or hood are made to revolve. 
Steam is also frequently employed for this purpose, but more commonly 
water, as this agent is more generally at hand to meet the primitive wants 
of man, before an advanced stage of the arts introduces the use of steam- 
mills, and the costly machinery necessary in the application of this power. 
1. VerticaL Water-WHEELS. 
Vertical water-wheels are those in which the shaft of the wheel is hori- 
zontal. Those wheels in which the force of the moving water is communi- 
cated to the buckets beneath the wheel are called wnder-shot wheels ; and 
those which are driven only by the weight of the water which is poured 
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