256 , DAIRYING. 



It is well to have a waste-pipe connected to the steam-pipe at 

 a point just above the engine- valve, in order that the water 

 which has collected in the steam pipe may be blown out before 

 opening the steam-valve. 



After the engine has been in operation for a minute or two 

 the waste-valves should be closed. 



Horse-power. — The horse-power of an engine may be calcu- 

 lated by means of the following formula: 



H. P. = ^'■'"^ 



33000 



H. P. = horse power; 



P = mean effective pressure in the cylinder; 

 Z, = twice the length of the stroke, in feet; 

 a = area of piston in square inches; 

 n = number of revolutions per minute. 



ON THK PRESERVATION OF MILK AND CREAM 

 BY HEAT. 



By Dr. H. L. Russell, of Wisconsin Experiment Station, 

 Author of " Dairy Bacteriology". 



On account of the innumerable bacteria that gain access 

 to milk during the process of milking, and subsequent to 

 that time, and the rapid increase of the same in this nutri- 

 tious fluid, this material universally undergoes fermentative 

 changes, the rapidity of which is largely dependent upon 

 the surrounding temperature. To increase the keeping 

 quality of milk, it is necessary to annihilate these bacteria 

 or keep them under influences unfavorable to their growth. 



Heat has been found to be the most efficacious agent in 

 preserving milk in its natural condition. It is applied in 

 two ways, viz., i. Pasteurization, where the milk or cream 

 is heated for a short time (20-30 min.) at a temperature near 

 the coagulating point of the proteid constituents of the milk 

 (i50°-i6o° F.). 2. Sterilization, where the temperature ap- 

 proximates or exceeds the boiling-point and is applied for 

 a longer time. 



The object in both cases is to kill the bacteria present in 

 the milk. 



