12 



per imitation of the ground vapour in tropical climates, 

 inasmuch as the soil under those circumstances will 

 not in general be much above 90 or 100 degs. 



And here it is that we fear the Polmaise system 

 may prove in part a failure, for with the great increase 

 of heat will be required a corresponding increase of 

 atmospheric moisture, otherwise vegetation will be 

 plundered. It remains to be seen, therefore, whether 

 this amount of moisture will not prove too great for 

 general purposes, and whether it be of a healthy cha- 

 racter. 



The best and most simple mode with which we are 

 acquainted is to provide a cemented gutter in front of 

 the house ; at least, where hot-water pipes are used. 

 We have had this mode in use for three years, and 

 find it perfectly satisfactory. The bottom or return 

 pipe rests on the bottom of this trench or gutter, and 

 by having a tap with a short leaden pipe fixed at the 

 furthest end of the house, together with a reservoir 

 which feeds the boiler at a higher level than the tap, the 

 bottom or return pipe, whenever necessary, can be co- 

 vered with water in less than half an hour. Now the re- 

 turn pipe, as is well known, is of a lower temperature 

 than the flow or advance pipe, and the amount of at- 

 mospheric moisture thus produced, is sufficient for 

 any tribe in cultivation. In fact, it is seldom the full 

 powers of this mode of producing moisture are put 

 in requisition. By admitting the cold or fresh air at 



