30 EVAPORATION. 



of the reservoir above the surface. It would be 

 needful to make the apparatus of some incorrodible 

 material ; — gutta percha, for instance, for the tube, 

 with a nozzle of glass ; — as metals would be acted 

 on by the sea- water, and form noxious oxides. The 

 water might either be carried up to the reservoir, or 

 pumped up by an obvious extension of the apparatus. 



Such a modification would doubtless be as 

 efEcient as it would be elegant. The constant, or 

 at least frequent, dissemination of the water through 

 the air would keep the whole volume in agreeable 

 coolness, as well as maintain its sparkling clear- 

 ness and purity. 



In a well-regulated Tank, however, none of 

 these modes are necessary. My oldest reservoir, 

 which has been in constant occupation for more 

 than two years, never has any artificial aeration, 

 except an occasional syringing, and that is often 

 intermitted for months together. The surface is 

 now and then agitated with a stick, and broken 

 by the addition of fresh water to supply the loss 

 by evaporation, and this is all the external aid it 

 receives. Yet the water generally maintains the 

 most crystalline transparency and purity. 



Evaporation. — If the Tank remain habitually 

 uncovered, or protected only by a coverlid of mus- 

 lin, daily evaporation will soon reduce the volume 

 of the water, and increase its specific gravity. The 

 pure water alone rises in vapour, the various salts 

 held in solution remaining the same in quantity, 

 though the water should be reduced to half its 

 original bulk. It is therefore needful that additions 

 of pure fresh water (not 56a-water) be made from 

 time to time, to replace the loss by evaporation. 

 Distilled water is of course the best, but, practically, 



