Another type of apparatus, developed by Wallace and Tiernan,[A] is shown in Figs. 6 and 7. The gas under the pressure indicated by the tank pressure gauge (Fig. 6) passes into the pressure compensating chamber, which maintains a constant drop in pressure across the chlorine control valve, through the check valve, and into the solution jar after measurement in the pulsating meter. The water required for dissolving the chlorine enters the jar through the feed line and check valve and the solution passes along the feed line after being water sealed in a special chamber. The meter is a volumetric displacement one and is regulated by observing the number of pulsations per minute. Each pulsation corresponds to 100 milligrams or 0.00022 pound of chlorine; diagrams for converting pulsations per minute into weight per twenty-four hours are usually provided with the apparatus. This type of meter is suitable for quantities between 0.1 and 12 pounds per day and possesses the distinct advantage of enabling the operator to see the actual delivery of the gas. [A] Manufactured by Wallace and Tiernan Co. Inc. N. Y. [Illustration: FIG. 7.--Manual Control Chlorinator, Solution Feed, Type B.] The quantities of gas exceeding 12 pounds per day the type shown in Fig. 7 may be used. The gas from the control valve passes through a visible glass orifice which is connected with the manometer. This manometer, or chlorine meter, contains carbon tetrachloride and is graduated empirically in terms of weight of chlorine per unit of time. A suitable gauge indicates the back pressure thrown by the check valve and registers the same pressure as the tank gauge when the flow of gas is stopped. The gas passes into the glass cylinder where it is dissolved in water and passes out by the feed pipe. The most accurate range of the orifice type is from 1-6, i.e. if the minimum graduation on the scale is 10, the maximum is 60. If quantities less than the minimum graduation are desired, a smaller orifice with its corresponding scale can be substituted in a few minutes. These types are manually controlled, but automatic control types, to meet almost any condition, can be obtained and are in use in many cities. In some instances (dry-feed types) the chlorine gas is not dissolved in water prior to addition to the water requiring treatment but is carried to the point of application as a dry gas and enters the water through a diffusion plate made of carborundum sponge. The sponge becomes saturated with water because of the capillary action of the carborundum upon the water. The pressure of the chlorine in the feed pipe forces the gas through the diffuser in the form of minute bubbles which become saturated with moisture. On meeting the water they immediately go into solution and no gas escapes.