10 BULLETIN 115, U. S. DEPARTMENT OF AGEICULTUEE. 



It is fast, permits accurate and close adjustment, and the position 

 of the lever operating the j)inion jDermits a man to use his weight 

 rather than his strength in adjusting the gate. The main disad- 

 vantage occurs in case of a broken casting, as no makeshift can be 

 substituted easily, and the gate may be out of use until a new casting 

 can be secured. This is one of the types of gate stem held by a pawl, 

 and as such is criticized for the reason that the space between the 

 notches controls the steps between the possible positions of the 

 shutter. 



As to the comparative advantages of wood, cast-iron, and sheet- 

 steel gates, wood, of course, is the cheapest in first cost. It may be 

 cut and adjusted easily to suit a gate structure that has settled or 

 otherwise gotten out of line ; it is elastic and will yield to prying where 

 a cast gate will break. Ice does not freeze as tightly to wood as it 

 does to iron or steel. On the other hand, the life of a timber shutter 

 is much less than that of a metal gate and, as ordinarily constructed, 

 it is less water-tight. Ice bulges wooden gates out of shape and will 

 spring a sheet-steel shutter, but the latter can be pounded back into 

 shape. A cast gate neither bulges nor springs but simply breaks. 



Cast and sheet-metal gates cost more in the first place, but last a 

 lifetime unless broken. Silt does not stick tightly to metal, especially 

 a smooth, vertical side of sheet metal ; such gates are practically water- 

 tight, present a better appearance, and for this reason add to the 

 project from a land-selling standpoint. The disadvantages are that 

 they are liable to rust shut and break unless faced with brass. When 

 they break the repair must come from a distance, taking time and 

 money, while wooden gates can be repaired on the ground with 

 lumber that can be obtained at any town. A cast gate is more easily 

 broken by the action of a crowbar on a stuck gate or by a log in 

 the stream than is either a sheet metal or a wood shutter. 



PAVING AND RIPRAP. 



The channel immediately above large gate structures may be eroded 

 by the action of racing water, while new banks below nearly all 

 structures become badly washed by the eddies of the issuing stream. 

 The entry channel may be protected by a lining of concrete 3 to 6 

 inches thick or by a riprap of hand-laid bowlders or broken stone. 

 This is sometimes grouted with cement mortar, forming a compara- 

 tively smooth, solid surface. 



The erosion below a structure is far more difficult to handle than 

 that above it and is more liable to cause the failure of the structure 

 unless watched and checked in time. A short, smooth lining is only 

 partially successful, as the high velocity in the water is not mate- 

 rially diminished and the erosion is only postponed until the water 



