This thermostat, as it is called, was placed inside of the oven. Heat causes brass to expand more than iron and therefore when the temperature reaches a certain height the thermostat curves, so as to break the contact with _C_, and the lamps go out. When the temperature falls a little the thermostat straightens until contact is again made with _C_. _C_ is a screw and can be made to advance or recede in its socket _E_, so that the temperature of the oven may be maintained at any point desired. The wire of the screw _C_ extends to the outside of the oven, where it carries an index, _D_, over the face of a dial, as shown in Fig. 178. [Illustration: Fig. 178] The cook may set this index at any desired degree, and the lamps will indicate when that degree has been reached. The thing to be baked is then put inside and the clock, illustrated in Fig. 175, is set so as to warn when the time is up. The electric spark which occurs when the thermostat breaks contact with _C_ causes the metals to corrode at that point, and corroded metals are poor conductors. This corrosion is due to the oxygen of the air. There is one metal--the expensive platinum--which is not corroded by the electric spark. We drilled small holes in the end of the screw _C_ and in the brass strip and pounded into these holes little pieces of platinum wire. Harold said he felt like a dentist filling a tooth. This furnished good, clean contact at all times. It takes a long time to heat up the brick oven, but it holds its heat a long time and makes an excellent fireless cooker after the lamps are turned out. It does not allow heat to escape into the kitchen, which makes it a comfort in our summer cottage. We are all becoming daft on _slowly cooked_ food--a sort of ripening process which gives time for the chemical changes to take place and develops the finest flavours of the food. XXI ELECTRIC WAVES Much has been said about bringing young people up to do what they don't like to do so as to make them strong and virtuous. My own life has always been guided by a different principle. It is: _Find something worth while which you will enjoy doing, and do it with your might._ I am bringing up my boy on the same principle. In September we have a real desire to get back to our work in the city, and in June we have an eager longing for the occupations of Millville. I am not aware that there is any part of my work which I would like to be relieved from, and Harold and his mother said that they were now ready to return to the city apartment with real pleasure for a winter.