948 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



taut pouits, so that ou the heat-engine side the hot steam passes into 

 the warmer and issues from the colder port, instead of the same chan- 

 nels being used alternately for the 

 entrance and exit, as in a recipro- 

 cating engine. 



Third. The steam or gas admit- 

 ted does not exert its pressure on 

 the whole area of the piston at 

 the initial point, but the surface 

 increases up to the half-stroke 

 and then diminishes, so that at 

 the end of the stroke or point of 

 maximum pressure, in forcing out 

 a gas, the resisting surface is re- 

 duced. By this contrivance, the 

 strain and friction are controlled. 

 Fourth. Slow valve-motion, the 

 valves moving only once in a com- 

 plete revolution. 



Fifth. Permanent opening of 

 the exhaust of the engine, so as 

 to expel the entire contents of the 

 cylinder, whether liquid or gas- 

 eous. 



Sixth. Absolute tightness, all 

 parts being pressure-packed. 



Seventh. The rotating instead 

 of the reciprocating motion of the 

 shaft causes a travel of but few 

 feet per minute and admits of ef- 

 fectual packing. 



Eighth. Placing the engine and 

 pumps on the same shaft obvi- 

 ates severe strains through many 

 joints, such as gibs, cotters, brasses 

 and crosshead, crank-pin, crank-shaft, &c. Moreover, the pistons can 

 be so arranged that the maximum power is exerted at the moment of 

 maximum resistance. 



l^mth. A very important feature in gas-pumps is great length of 

 stroke, and this is admirably secui^ed in the rotary pump. In one revo- 

 lution the travel is a little over three times the diameter of the cylinder, 

 whereas in a reciprocating pumj) it can only be twice the length of the 



erank. 4 



E.— EEFEIGEEATOES AND CONDEXSEES. 



The annexed drawings, of tubular sections of my freezing machines, 

 indicate the manner in which I obtain the maximum conducting surface 



