MAY. 95 



retained by the surface vegetation and slowly yielded up 

 by the never-failing streams and rills, it now rushes off the 

 sloping ground, carrying away the best of the soil into the 

 valleys, and it does damage by its unrestrained vehemence. 

 Hundreds of acres of land formerly covered with noble 

 bush, the soak age from which formerly fed the northern 

 reservoir, have been cleared during the last two or three 

 decades to make way for grass fields, which cannot retain 

 the moisture which the forest formerly held. And to 

 make matters worse, houses have been erected in this 

 catchment area and the drainage from these must find 

 its way sooner or later into the town water supply. It 

 is wonderful how complacently we accept all these con- 

 ditions without so much as raising a finger to alter them. 

 Give us a plague scare and we begin to bustle round and 

 find out the objectionable features of our neighbours' back 

 yards, but as to supplying the citizens with clean water 

 why, that is a totally different matter. 



There is one kind of weather of a very trying and 

 objectionable character which is experienced in all the 

 open country on the east side of this island, from South- 

 land to Marlborough, but which we get in a very mild form 

 in Dunedin, thanks to the hilly environment. I refer to 

 the hot nor'-w r esters, which in the Canterbury Plains are 

 a perfect abomination, sweeping the very soil off the 

 ploughed fields and parching the vegetation. The explana- 

 tion of these hot winds is very simple and is familiar to 

 every student of physical science, but is probably not 

 popularly known. 



When a north-westerly wind which has traversed the 

 Tasman Sea reaches the West Coast of this island, 

 saturated with moisture, it meets with a mountain range 

 6000 or more feet in height, against which it is forced and 

 which it has to surmount. In accordance with a well- 

 known physical law, the energy required to raise the mass 

 of air over these mountains causes a very considerable 

 expenditure of heat, or, as we would otherwise express it, 

 a fall in temperature, and this results in a great condensa- 

 tion of moisture. But when water vapour condenses a 



