Many would find, on examination, that some of their cows, though giving a good quantity, were comparatively worthless to them. Such was the experience of John Holbert, of Chemung, New York, who, in his statement to the state agricultural society, says: “I find, by churning the milk of each cow separately, that one of my best cows will make as much butter as three of my poorest, _giving the same quantity_ of milk. I have kept a dairy for twenty years, but I never until the past season knew that there was so much difference in cows.” [Illustration: Fig. 72. Lactometer.] The simplest form of the lactometer is a series of graduated glass tubes (Fig. 72), or vials, of equal diameter; generally a third of an inch inside, and about eleven inches long. The tubes are filled to an equal height, each one with the milk of a different cow, and allowed to stand for the cream to rise. The difference in thickness of the column of cream will be very perceptible, and it will be greater than most people imagine. The effect of different kinds of food for the production of butter may be studied in the same way. This form of the lactometer was invented by Sir Joseph Banks. Various means are used for the preservation of milk. One of these is by concentrating it by boiling. Where this is followed, as it is by some dairymen, as a regular business, the milk is poured, as it comes from the dairy, into long, shallow, copper pans, and heated to a temperature of a hundred and ten degrees, Fahrenheit. A little sugar is then mixed in, and the whole body of milk is kept in motion by stirring for some three or four hours. The water is evaporated, leaving the milk about one fourth of its original bulk. It is now put into tin cans, the covers of which are soldered on, when the cans are lowered into boiling water. After remaining a while, they are taken out and hermetically sealed, in which condition the milk will keep for months. Concentrated milk may thus be taken to sea or elsewhere. Another form is that of solidified milk, in which state it is easily and perfectly soluble in water; and when so dissolved with a proper proportion of water, it assumes its original form of milk, and may be made into butter. A statement by Dr. Dorémus, in the New York _Medical Journal_, explains the process, as follows: