_Position of the Thermometer._ In figuring on the proper temperature at which to run the incubator, the thermometer should be so placed that the bulb is on a level with the top of the eggs, preferably touching a fertile egg. If the thermometer bulb rests on an infertile egg the temperature recorded will be lower than the actual temperature of fertile eggs in the later stages of incubation, due to the animal heat of the developing embryos, with the result that the machine would be operated at too high a temperature. _Testing._ It is common practice to make only one complete test. This is done on the evening of the fifth day. Testing may be done by means of an ordinary candling device such as is used with hens' eggs, each egg being examined separately. To save time a piece of apparatus may be used which is simple in construction and which simplifies the process of candling considerably. This may be termed a testing table. It consists of a table the same width as an incubator tray and longer than the tray. In the table there is an opening the size of a row of eggs and beneath this are placed several electric light bulbs with reflectors back of them so as to throw the light up through the eggs. By sliding the tray along the table each row of eggs is brought over the lights and their condition can be quickly noted. At this test all the infertile eggs are taken out as well as any eggs in which the germs have died. The infertile eggs after a careful retest are then packed in cases and sent to market where they are usually sold to bakers as tested eggs. While no second test is made of the eggs left in the machines the experienced incubator operator is constantly on the watch for and is constantly removing any eggs which die at a later time. To the experienced eye the color of the egg indicates that it has died as it takes on a sort of pinkish or darkish tint. Duck eggs after they die will spoil very quickly and must be removed promptly as the odor which they throw off is very strong and will prove harmful to the other eggs. The inexperienced operator can readily locate dead eggs by smelling over the tray. [Illustration: FIG. 21. Interior of house for breeding ducks. Notice the heavy bedding and the feeding track. (_Photograph from the Bureau of Animal Industry, U. S. Department of Agriculture._)] [Illustration: FIG. 22. Incubator cellar on large duck plant. Trays of eggs set out to turn and cool. (_Photograph from the Bureau of Animal Industry, U. S. Department of Agriculture._)] _Turning the Eggs._ The eggs are neither turned, cooled nor otherwise disturbed after they are put in the incubator until after they are tested on the fifth day. From this time on they are turned twice a day, morning and night, until they begin to pip.