1S4 ROYAL SOCIETY OF CANADA 



water passing through the wheel. The gate has rods attached to it — not 

 shown — which pass out into the generator room and are connected with 

 the controlling mechanism. 



D is the turbine or " runner " as it is called. It is keyed solidly to 

 the shaft, which projects into the generating room — if this happens to 

 be a hydro-electric station. A coupling unites this shaft to the gener- 

 ator shaft, or a pulley mounted upon this shaft will drive machinery of 

 any character. 



E is the draft tube through which the water flows or falls into the 

 tail race, and being air-tight it enables the turbine to operate under the 

 total head of a water fall even though the turbine is not situated at the 

 exact bottom of the fall, 



F is the tail race which is simply a channel for the purpose of car- 

 rying off the water after it has done its work of driving the machinery. 



A power plant of this character suffers from frazil in the following 

 manners : — 



(1) The openings in the rack get clogged, and, as the supply of 

 water is reduced, so is the output or capacity of the plant les- 

 sened. When rack-clogging once begins a complete shut down is 

 usually inevitable within a very short period of time. Some institutions, 

 believing a shut down unpreventable in any other way, entirely remove 

 the rack just before winter sets in. Their water-wheels then run 

 the risk of being destroyed by floating debris, but this risk is, in 

 their opinion, preferable to the certainty of rack clogging and a shut- 

 down. Hand and mechanical rakes are used for the purpose of removing 

 frazil from racks. The raking simply consists in scraping the frazil from 

 the bars and, when it is scraped off, the current carries it against the 

 rack again or into the wheel-pit. If a side sluice exists near the end of 

 the rack much of the frazil may be floated over it. 



Some idea of the amount of work required to keep a rack open dur- 

 ing an attack of frazil may be gathered when I say that the combined 

 efforts of two motor-driven rakes and a gang of willing able-bodied 

 hand-rakers, placed so close together that they elbow each other, are 

 often unable to keep a rack half open. Fig. 4 shows the front of a rack 

 with a motor-driven rake at the left of the picture, and Figs. 5 and 6 

 give only a faint idea of the amount of frazil which is sometimes en- 

 countered. 



(2) When the rack is kept open by raking, the next difficulty that 

 is encountered is the sticking or freezing of the water wheel's controlling 

 gate. The frazil clings to the chutes and to the exposed part of the gate 

 and it cements the gate so firmly to the parts of the wheel case which 

 surround it that the gate cannot be moved. The wheel in this condition 



