When cream, especially cream in which incipient lactic fermentation has been set up, is subjected to agitation in a churn under proper conditions of temperature the particles of butter therein contained are collected into masses so that the butter can be separated from the residual liquid. This process is technically called churning. The domestic churn in its simplest form is perhaps well known to almost everyone, especially those who have lived in the country. In the domestic manufacture of butter the cream is collected and set aside until sour, that is, until lactic fermentation has set up. When this is sufficiently advanced the cream is placed in a churn, the simplest form of which is a wooden, cylindrical vessel of appropriate size, being much longer than its horizontal diameter. The churn is provided with a dasher, namely a perforated wooden disk with a handle which passes through a hole in the cover. When the churn is charged the butter is produced by agitation with the dasher. In winter time warm water is added to the mixture in order to raise the temperature to the proper gathering point of butter, namely 65 to 70 degrees F. For the same reason cold water is added in the summer time. The art of the dairy maid is shown in the proper regulation of the temperature to secure the best results. When the cream is properly ripened and the temperature is suitable the gathering of butter will be accomplished in from twelve to thirty minutes. In unfavorable conditions the duration of churning may be for a much longer period. [Illustration: FIG. 20.--POWER CHURN READY FOR USE.--(_Courtesy of the Bureau of Animal Industry._)] In dairies and large establishments churning is accomplished by machinery with very different mechanical appliances, but the principle which underlies the process is the same as those outlined above. The accompanying figures illustrate the process of churning by mechanical means in a modern dairy (Figs. 20 and 21). [Illustration: FIG. 21.--POWER CHURN, OPEN.--(_Courtesy of the Bureau of Animal Industry._)]