At the Lawrence Experiment Station of the Massachusetts State Board of Health as early as 1889 it became necessary to express the turbidities of various waters approximately, and the very simple device of sticking a pin into a stick, and pushing it down into the water under examination as far as it could be seen, was adopted. Afterwards a platinum wire 0.04 of an inch in diameter was substituted for the pin, and the stick was graduated so that the turbidities could be read from it directly. The figures on the stick were inversely proportional to their distances from the wire. When the wire could be seen one inch below the surface, the turbidity was reported as 1.00; when the wire could be seen two inches, the turbidity was 0.50, and when it could be seen ten inches the turbidity was 0.10, etc. This scale is much more convenient than a scale showing the depth at which the wire can be seen; and within certain limits the figures obtained with it are directly proportional to the amount of the elements which obstruct light in the water. Thus, if a water having a turbidity of 1.00 is mixed with an equal volume of clear water, the mixture will have a turbidity of 0.50. Advantage is taken of this fact for the measurement of turbidities so great that they cannot be accurately determined by direct observation. For turbidities much above 1.00 it is very difficult to read the depth of wire with sufficient accuracy, and such waters are diluted with one, two, or more times their volume of clear water in a pail or other receptacle, the turbidity of the diluted water is taken, and multiplied by the appropriate factor. For the greatest accuracy it is necessary that the observations should be taken in the open air and not under a roof. They should preferably be made in the middle of the day when the light is strongest, and in case the sun is shining, the wire must be kept in shadow and not in direct sunlight. The turbidities of effluents are usually so slight that they cannot be taken in this manner; in fact, turbidities of less than 0.02, with the wire visible 50 inches below the surface, cannot be conveniently read in this way. For the estimation of lower turbidities a water is taken having a turbidity of 0.03 or 0.04 and as free as possible from large suspended particles. The turbidity of this water is measured by a platinum wire in the usual way, and the water is then diluted with clear water to make standards for the lower turbidities.