264 MADRAS FISHERIES BULLETIN VOL. Xllt, 



NOTE. 



A SIMPLE FORM OF SOLAR OVEN. 



(See paragraph 44 of Bulletin ''Remarks on Canning.") 



Description. — The " solar oven " or trap consists of a ivcll- 

 seusoncd teak-wood box (see A in appended sketch of vertical 

 section) of any convenient size, and about 8 inches in depth below 

 the glass sash, the inside being carefully blackened with a dull or 

 dead (matt) black, not a glossy black. The sash consists of a 

 series of glass panes (ordinary window glass) set just like an 

 ordinary window, but double, the panes being separated by a 

 distance of l^ to 2 inches ; see B in sketch of a vertical section. 

 This double sash is the important part of the oven, as it allows the 

 direct rays of the sun to pass freely, but intercepts the rays 

 radiated from the dull black interior, according to a well-known 

 fact in physics ; hence the heat is trapped in the oven. The reason 

 for a double sash is not only to duplicate the above effect but to 

 insulate in some degree the heated air in the oven at, say, 250" to 

 3C0° F. from the external air at, say, 90'; with a single sash the 

 difference between the temperatures on each side of the single 

 pane would be so great that much heat would be lost not by 

 radiation but by conduction ; the stagnant air-cushion between the 

 two sets of glass panes, to some extent prevents loss by conduction. 

 All wood must be thoroughly seasoned so that it may not warp or 

 shrink. Obviously, also, the double sash must fit accurately into 

 the box or there will be great loss by leakage of hot air. 



2. Insulation.— To prevent further loss by conduction through 

 the walls of the box, this is inserted in an outer box with 3 or 

 4 inches of insulating packing (sawdust, paddy husk, etc.) 

 between it and the oven ; this outer box may be a rough packing 

 case. 



3. Reflectors.— To increase the internal temperature of the oven, 

 reflectors may be placed outside the oven at such an angle that 

 the sun's perpendicular rays may be thrown into the box ; this has 

 not been fully worked out as the heat obtained by a rough arrange- 

 ment was sufficient for the cannery purpose (baking or" stoving " 

 lacquered tin ware) ; a couple of small wings of tin plate at the 

 proper angle sufficed. 



