=217. Water Seal.= Water-seal canners are like the water-bath canners, except that the cover has a flange on it, the depth of the boiler, and about two inches from the sides of it. This makes a jacket of water between the flange and sides of the canner. This causes the temperature inside to rise about two degrees above the ordinary temperature of boiling water. Food can be sterilized in a little shorter time in this canner than in the ordinary water bath. It is as important that the water entirely cover the jar in this canner as in the water bath. =218. Pressure Canners.= Pressure canners are made very strong and have covers which fit tight, making it possible to raise the temperature in them considerably above the boiling temperature of water, so the food may be sterilized in a very short time. [Illustration: FIG. 121. Pressure canner showing pet cock.] The pressure canner has either a rack or a perforated pail on the inside to raise the jars from the bottom as in other canners. It is also fitted with a steam gage which registers the pounds of pressure in the canner. Five to fifteen pounds pressure is used for canning. The amount of pressure needed and the time of sterilizing depends on the organism present. A higher pressure is an indication of a higher temperature in the canner. After the jars are filled and put in the canner, the cover is fastened down tight by thumb-screws. There is a pet cock which is kept open when the canner is first heating, to let the air be forced out by the first steam which forms. As soon as the steam begins to escape, the pet cock is closed and the temperature inside of the canner begins to rise above the temperature of boiling water (Fig. 121). On the canner is a safety valve which is set so that the instant a certain number of pounds of pressure is reached, it is lifted up by the steam. Some of the steam then escapes, thus preventing the pressure in the canner becoming so great that there is danger of its exploding. =219. Use of the Canner.= Water is put into the canner to reach to the bottom of the rack. The jars are filled according to canning directions and are set in the canner. When the jars are in, the cover is adjusted to the canner and screwed on tight so that no steam will escape between the cover and the canner. The pet cock is left open until steam begins to escape thru it as the canner is heating on the stove. When steam begins to come, the pet cock should be closed, and the steam-gage hand then begins to turn, indicating that the pressure in the canner is rising. When the steam-gage reaches the point desired, the safety valve is adjusted so that the steam will escape should the pressure continue to rise. Until the operator knows where to set the weight to the safety valve, leave it well out to the end of the rod until the pressure in the canner has reached the desired point. Then move the weight to the point on the arm of the valve which will just keep in the steam.