18 BULLETIN 890, U. S. DEPARTMENT OF AGRICULTURE. 
Fig. 10. — Open or. surface milk cooler. 
of milk is retarded in its course by some form of obstruction, such as 
baffle plates, or in some types it passes through a series of winding 
pipes. It is supposed to take half an hour for the milk to pass 
through the holder. Some continuous holders consist of a series of 
two or more tanks, the milk flowing from one tank as soon as it is 
full to the next, and half an hour is required for it to go through 
all the tanks. 
Good results have been obtained with many holders of the con- 
tinuous type. The danger with such holders, however, is that some 
particles of the milk may not be held so long as others and that the 
milk may be forced through the apparatus and none of it held the 
proper length of time. Tests made on several retarders of one type 
have shown that some of the milk was held only 10 minutes. Tests 
should be made on all such apparatus to ascertain whether the milk 
is being properly held, and retarders that do not do accurate work 
should not be used. 
Of 237 plants surveyed in the year 1916, 15 were using the " flash " 
system. 21 were using " retarding " systems, and 198 were using 
positive holders. 
COOLERS. 
In general there are two common types of coolers — the open- 
surface tubular cooler and the internal-tubular cooler. With the 
open-surface cooler the milk passes over the tubes in a thin film. 
The cooling medium passes through these tubes on the inside. There 
is usually water in the first sections and brine or direct-expansion 
