Lactometer is an instrument for testing the quality of milk. One form of lactometer is a hydrometer to which is attached a scale about five inches long, the zero of which marks the point to which it just sinks in water. Another point on the scale corresponds to an average quality milk, and according as this mark sinks below or rises above the level of the milk, the milk is said to be more or less diluted with water. The instrument is not scientifically accurate, as the addition of either cream or water would cause the instrument to sink. Another form is a simple glass tube graduated in 100 parts. New milk is poured in and left to stand. The cream rises, and its height gives the number of parts of cream in 100 parts of milk. Sets of these tubes are used by farmers for comparing the quality of the milk given by different cows.