[Illustration: FIG. 122. Device for sealing tin can.] Be sure the cover is properly adjusted. Be sure to exhaust the air from the canner before closing the pet cock. Keep the fire so that the desired pressure will be maintained without the escape of steam from the safety valve. When steam escapes from the canner thru the pet cock at a rapid rate, it may cause liquid to flow out of the jars. Be certain to let the canner cool until the indicator on the steam-gage has reached zero before opening the canner. When the indicator points to zero, open the pet cock. If a heavy stream of steam starts to escape from it, close it again and wait a few minutes longer. Test again by opening the pet cock; if a very little stream of steam escapes, leave the pet cock open and wait until steam has stopped escaping from it. Now loosen the screws holding the cover in place. Partially loosen each screw. When this is done, fully loosen all and lift off the cover. These precautions are taken to prevent the operator from being burned by steam or getting hurt by the cover being lifted by the steam. It also prevents the breaking of glass jars due to sudden pressure changes. Never let the canner cool so long before the pet cock is opened that air will rush into it, due to the vacuum which is sure to form when the steam is cooled if the pet cock is not opened. Such a condition may break the jars. [Illustration: FIG. 123. Dryer.] Tin cans are sealed with a device (Fig. 122) which folds the edge of the cover over the top of the can so tightly it will not leak. =220. Dryers.= Dryers are devices to hold the food being dried in a thin layer so that the air can be circulated thru it freely. Sometimes they are devised to direct currents of air thru the drying material. If the air is heated, the drying is hastened (Fig. 123). A sieve on which food is spread hung above the stove is a simple drying device and one of the most practical for home use. The heat currents rising from the stove pass thru this and dry the food. Many dryers are constructed on this same principle, having a heating unit below and trays of food above. These trays have to be shifted from time to time, as the moisture from the lower ones rises with the heat to the upper trays, thus retarding their drying. The top trays, if too numerous, are useless on this account. Two or three seem to be all that can be used with advantage at one time in home dryers, the some machines are made with many more. Another type of dryer has a fan device in it which forces the air thru at a faster rate than would be accomplished by heat alone. Such air should pass thru a strainer. Ordinary air, even when drawn from a clean room, carries much dust with it, and if the dust is not strained out previously, it is strained out by the food. This injures the quality of the product. Large commercial dryers provide such a strainer.