the pound or by the gallon, but by the amount of butter-fat contained in
each pound or gallon of milk. A gallon of milk containing four and a
half per cent of fat will consequently be worth more than a gallon
containing only three per cent of fat. So it may happen that a cow
giving only two gallons of milk may pay a butter-maker more than a cow
giving three gallons of milk. Of course it is easy to weigh or measure
the quantity of milk given by a cow, and most milkers keep this record;
but until recent years there was no way to find out the amount of fat in
a cow's milk except by a slow and costly chemical test. Dairymen could
only guess at the richness of milk.

In 1890 Dr. S. M. Babcock of the Wisconsin Experiment Station invented a
wonderful little machine that quickly and cheaply measures the fat in
milk. Few machines are more useful. So desirous was Dr. Babcock of
helping the farmers that he would not add to the cost of his machine by
taking out a patent on his invention. His only reward has been the fame
won by the invention of the machine, which bears his name. This most
useful tester is now made in various sizes so that every handler of milk
may buy one suited to his needs and do his own testing at very little
cost.

The operation of the machine is very simple. Suppose that the members of
the class studying this book have been asked to take a Babcock machine
and test the milk of a small herd of cows. They can readily do so by
following these directions:

While the milk is still warm from the first cow to be tested, mix it
thoroughly by pouring it at least four times from one vessel to another.
A few ounces of this mixed milk is then taken for a sample, and
carefully marked with the name of the cow. A number is also put on the
sample, and both the cow's name and the number entered in a notebook. A
small glass instrument, called a pipette, comes with each machine. Put
one end of the pipette into the milk sample and the other end into the
mouth. Suck milk into the pipette until the milk comes up to the mark on
the side of the pipette. As soon as the mark is reached, withdraw the
pipette from the mouth and quickly press the forefinger on the mouth
end. The pressure of the finger will keep the milk from running out.
Then put the lower end of the pipette into one of the small long-necked
bottles of the machine, and, lifting the finger, allow the milk to flow
gently into the bottle. Expel all the milk by blowing throu