The percentage only of each ingredient the scale is composed of is given, as it cannot be told how much water was evaporated to leave this amount of solid matter.
THE WATER METER.
Water meters, or measurers, are constructed upon two general principles: 1, an arrangement called an “inferential meter” made to divert a certain proportion of the water passing in the main pipe and by measuring accurately the small stream diverted, to infer, or estimate the larger quantity; 2, the positive meter; rotary piston meters are of the latter class.
Fig. 603.
Figs. 604, 605.
The distinctive difference between the two is, that the positive meter measures water by means of a chamber alternately filled and emptied. In most of these the flow of water ceases when, by any derangement, the motion of the piston is interrupted. But neither the motion nor the stoppage of the inferential meter has any effect upon the water delivery, so that at times a large amount of water may pass unrecorded. Another important mechanical difference is that the motion of a piston meter should be slow, while that of the inferential wheel is, and must be, rapid; this has much to do with their relative durability.