444 POPULAR LECTURES AND ADDRESSES. 



mills as hitherto made arc very costly machines ; 

 and it does not seem probable that, without in- 

 ventions not yet made, wind can be economically 

 used to give light in any considerable class of 

 cases, or to put energy into store for work of other 

 kinds. 



Consider, lastly, rain-power. When it is to be 

 had in places where pow r cr is wanted for mills and 

 factories of any kind, water-power is thoroughly 

 appreciated. From time immemorial, water-motors 

 have been made in large variety for utilising rain- 

 power in the various conditions in which it is pre- 

 sented, whether in rapidly-flowing rivers, in natural 

 waterfalls, or stored at heights in natural lakes or 

 artificial reservoirs. Improvements and fresh in- 

 ventions of machines of this class still go on ; and 

 some of the finest principles of mathematical 

 hydrodynamics have, in the lifetime of the British 

 Association, and, to a considerable degree with its 

 assistance, been put in requisition for perfecting the 

 theory of hydraulic mechanism and extending its 

 practical applications. 



A first question occurs : Arc we necessarily 



