6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



mirror. In the case of the cooker, and also of the power flash boiler, 

 soon to be described, the absorber of rays is made as small in diameter 

 as possible in order to reduce heat losses, so that the temperature 

 may run high. In the solar water distiller, however, the temperature 

 cannot exceed the boiling point of water. With a vacuum jacket 

 surrounding the focus tube, heat losses at that temperature are small 

 per unit area. Hence the focus tube is made much larger in diameter 

 in (jrder to provide freer escape for steam. This requires a larger 

 vacuum jacket than in the devices for cooking and for power. 



I pour the water to be distilled into a vessel supported behind the 

 mirror and nearly at the level of the upper end of the mirror. A long 

 snout runs from the bottom of the water vessel down behind and 

 parallel to the mirror, and, bending at right angles, comes up to join 

 the focus tube of copper, which is blackened outside to absorb solar 

 rays. Thus the water flows by gravity from the vessel to an equal 

 height within the focus tube. Within this snout and focus tube is a 

 smaller tube for steam. It extends from above the level of the water 

 in the vessel to above the level of the mirror in the focus tube. It is 

 open to the atmosphere above the vessel, and open to steam above 

 the water in the focus tube. A branch leaves the steam tube at its 

 lowest point, and passes sealed through the wall of the snout, so that 

 distilled water may drop from the steam tube into a receptacle 

 underneath. 



Only one difficulty is met with in this device. The steam must be 

 caused to escape by such a protected orifice that the surging, boiling 

 water within the focus tube does not ever reach that orifice to mingle 

 with the condensed steam. This is accomplished by a series of 

 umbrella diaphragms along the upper part of the steam tube, and by 

 using a diminished orifice, well shielded by a cap. 



The efficiency of the device is very high. The steam being con- 

 densed by flowing through the entering water, that water reaches the 

 lower end of the boiler tube at almost boiling temperature. Thus it 

 is only the latent heat of steam that must be provided by solar 

 radiation, and not the heat required to raise water to boiling. In 

 experiments made in Florida in March 1938, the stinking water of 

 Arcadia was distilled to perfect purity and odorlessness. Distillation 

 commenced within 5 minutes after the sun came out of a cloud. A 

 mirror of 1 1 square feet of surface distilled between 2 and 3 gallons 

 of water, entirely automatically in one cloudless day. 



