POPULAR LECTURES AND ADDRESSES. 



acres at 2/. or 3/. per acre. But the value of land 

 is essentially much more than its rental, and the 

 rental of land is apt to be much more than 

 2/. or 3/. per acre in places where 100 horse-power 

 could be taken with advantage from coal through 

 steam. Thus the question remains unsolved, 

 with the possibility that in one place the answer 

 may be one hundred horse-power ^ and in another 

 forty acres. But, indeed, the question is hardly 

 worth answering, considering the rarity of the 

 cases, if they exist at all, where embankments 

 for the utilisation of tidal energy arc practic- 

 able. 



Turning now to sources of energy derived from 

 sun-heat, let us take the wind first. When we 

 look at the register of British shipping and sec 

 40,000 vessels, of which about 10,000 arc steamers 

 and 30,000 sailing ships, and when we think how 

 vast an absolute amount of horse-power is de- 

 veloped by the engines of those steamers, and how 

 considerable a proportion it forms of the whole 

 horse-power taken from coal annually in the whole 

 world at the present time, and when we consider 



