'RKS1I)F.NTI.\L ADDKKSS SKC'TION .\. 



29 



nothing, and, when it has (U»nc its work, no i)rovision has to be 

 made for its removal. In the smaller sizes, the maehines are low- 

 in first cost; practically no attention is rerpiired in runninji^, and 

 maintenance costs are almost neijligible. ( )nly lii^ht foundations 

 are retjuired; they are easily erected, and hein,i;- _a^enerally of steel 

 construction, thev are capable of withstandinj^' heavy ^ijales, while 

 they will start up under light breezes. For pumping water and 

 other ptu-j>©ses in which tise of an intermittent character can be 

 made they are admirably ada])ted, but the power obtained musi 

 alwavs be small, as shown by Table ill. 



TA 13 LF-: III. 



Wind Velocity. 



A windmill 40 feet in diameter would give about 8 h.p., 

 but the cost would be about £50 per h.p. No windmill can be 

 relied upon for more than one-third of the total number of 

 hours in a year, and even then, the working hours are distri- 

 buted very irregularly. Some cheap method of storing power 

 is necessary to enable prime movers of such a fluctuating and 

 uncertain nature to compete with energy derived from fuel. 



The figures show the development of the primitive types 

 to the present form. 



\ 



Pig. 7. — Earliest tvpe of Windnii 

 Post '.Mill. 



ViG. S. — Early type of Tower Mill 



The post mill (Fig. 7), the earliest type, is still to be 

 found in Europe. The wooden house containing the machinerj 



