488 



habitants resorted to the expedient of constructing reservoirs 

 under ground, and in some instances they even went so far as 

 to conduct rivers league after league beneath the soil, in order 

 to obviate the evil results of evaporation : each village having 

 a well, by means of which they were enabled to draw up the 

 treasures of these subterraneous currents. In Constantinople 

 the Greek emperors built extensive arched reservoirs for hold- 

 ing water, and from which a considerable portion of the city 

 was supplied with this indispensable element. 



" To persons living in this moist climate it may appear of 

 very little importance to guard against the evaporation of 

 water, and the effects arising from it ; but to the philosopher, 

 who, by experiments in his laboratory, has made himself fa- 

 miliar with the power of evaporation ; or the traveller, who, 

 like Mr. Strutt, has seen in Australia evaporation carrying off 

 daily from reservoirs the water upon which his own life, and 

 that of his companions and cattle, depended ; and who has 

 marked the appalling rapidity with which it disappears, when 

 the air has become dry and parched from the great heat pre- 

 vailing ; — to such, I say, it must appear evident, that evapo- 

 ration may become so intense as to render nugatory all efforts 

 to preserve any large supply of water in open tanks or reser- 

 voirs, and thus prevent the colonization of countries in other 

 respects most desirable. Sir T. L. Mitchell observed the 

 thermometer at 126° in the shade, under the influence of the 

 hot dry wind of Australia. How rapidly must evaporation pro- 

 ceed, when water is exposed to such a wind ! 



" It is my object to show, that this great source of waste 

 can be effectually prevented, and that too by very simple 

 means ; and that the surface of reservoirs, tanks, water-holes, 

 and ponds, in the hottest climate and warmest weather, can be, 

 for the most part, preserved from the loss occasioned by eva- 

 poration. 



" The method which I propose for resisting the evapo- 

 ration of water could not have been either thought of or ap- 



