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plied before the present time, when a succession of discoveries 

 has rendered us masters of many vegetable substances, such 

 as Indian-rubber, gutta-percha, &c., which, after passing 

 through certain simple processes, may be made available for 

 rendering linen, cotton, woollen cloths, and canvass, when even 

 of the finest texture, impervious either to air or water. It is 

 by the help of such prepared canvass that the contrivance 1 

 am about to describe is to be carried into execution. 



" Those who are practically versed in this department of 

 art, will readily suggest what species of impermeable water- 

 proof canvass is best adapted in practice for accomplishment 

 of so desirable an end. For me it is sufficient to know that 

 such a material can be manufactured at a cheap rate, of a light 

 but strong texture, and in sufficient quantities to cover, as 

 with a carpet, any extent of water that may be necessary. 

 When a piece of water is to be protected from evaporation, its 

 water-proof carpet may be spread by the following simple 

 means: — at suitable distances from each other on the canvass, 

 and made of the same material, are to be inserted pouches 

 or bags, which, when it is wished to float the canvass 

 on the water, may be inflated with air in the usual manner; 

 when not in use, these bags need not, of course, be otherwise 

 than in their coUapsd condition. Let us suppose a piece of 

 canvass nine feet in breadth, and 150 in length, having three 

 bags in rows, twelve or fifteen feet distant from each other ; 

 let us suppose such a piece of canvass placed on the water of 

 a tank, it will then protect from evaporation a surface corres- 

 ponding to its own extent. Similar pieces might be attached, 

 by tying together their sides and ends, until the whole surface 

 was similarly protected. All this could be done without much 

 labour or trouble: the canvass could be carried in a boat, and 

 dropped from its stern, in the same manner as fishermen 

 drop their nets, the men inflating each row of pouches or 

 bags at the time it became necessary to cast them in ; ropes 

 could be also fastened to the extremities of the piece, so as to 

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