HOW THE SUN WARMS THE EARTH ABBOT 159 



for about a minute, awaiting the next pleasure of the little master 

 clock. The whole driving outfit required an outlay of about $15 

 and 3 days' work. It has operated perfectly satisfactorily for about 

 8 hours a day through several summers. 



The reservoir, 20 by 24 by 36 inches, was made of steel, with 

 welded corners. Two ovens, each 9 by 11 by I6I/2 inches, were let 

 in at the back. The reservoir stands on a platform about 6 feet 

 above the top end of the mirror. To conduct the heated oil, there 

 is a copper pipe li/4 inches in diameter, bent so as to pass from the 

 reservoir down under the mirror, returning in the focus of the sun 

 rays through the hollow trunnions. 



The reservoir and pipes are protected from loss of heat by brick- 

 work composed of diatomaceous earth. Such bricks are known by 

 the trade name " silocel." Only substances like cotton and silk-fluff 

 are less conducting, and their insulating advantage over the bricks 

 is very slight. As these bricks are perfectly fireproof, they are 

 highly suitable for use around an oil reservoir, heated nearly to 

 the limit of safe oil temperatures. 



It remains to speak of the coverings of the mirror, and of the 

 heater tube within it. In order to avoid cooling by air currents, 

 and to prevent the mirror surface from being fouled by dust and 

 other things, the whole top of the mirror frame is covered by sheets 

 of window glass. To prevent the loss of heat by air convection 

 from the very hot blackened heater tube, this metal tube is sur- 

 rounded by a glass tube, leaving an air space of about three-fourths 

 inch all around it. But the glass tube grows very hot, and itself 

 tends to cool, so that great losses of heat would still occur there. 

 This is hindered by a second concentric tube of glass, larger than 

 the first, and separated from it by the highest practicable vacuum. 

 In this way, the heated inner tube is, like the inner part of a thermos 

 bottle, prevented from cooling by a vacuum jacket. Of course, it 

 is not practicable to coat the inner glass with silver, like a thermos 

 bottle. Complete silvering of it would shut out the sun rays. How- 

 ever, the upper one-third of its surface is silvered, which helps 

 decidedly to prevent the escape of heat and hinders very little the 

 sun rays from entering. 



The whole cooker, with all of its coverings, is shown in plate 3. 

 Temperatures of 175° C. are readily attained. This is quite hot 

 enough to bake bread. 



Fruit is preserved with great ease. The prepared fruit is put 

 into glass jars, covered over with sirup, and left overnight in the 

 lower oven. In the morning, the jar covers are fastened down while 

 still hot. That is all ! Indeed, the cooker is highly convenient. 

 Foods may be prepared in a cool kitchen, having no fire. A beauti- 

 ful mountain view is seen as they are set into the ovens out of doors. 



