210 HYDRODYNAMICS. 



slope similar to that of the natural beach. Up this 

 slope the waves rolled, and became gradually spent as 

 they ascended, till they entirely died away. The 

 breach was effectually stopped, and this simple struc- 

 ture has ever since resisted the most violent storms of 

 the G-erman Ocean. 



SECTION V. 

 CONTENTS OF CISTERNS. 



Connected with the subject of hydrauUcs is the 

 collection and security of water falling upon roofs, in 

 all cases where a deficiency is felt by farmers m the 

 drought of summer. The amount which falls upon 

 most farm-buildings is sufficient to furnish a plentiful 

 supply to all the domestic animals of the farm when 

 other supplies fail, if cisterns large enough to hold it 

 were only provided. Generally speaking, none at all 

 are connected with barns and out-buildings, and even 

 when they are furnished, they are usually so small as 

 to allow four fifths of the water to waste. 



If all the rain that descends in the Northern States 

 of the Union should remain upon the surface without 

 sinking in or running off, it would form each year a 

 depth of about three feet. Every inch that falls upon 

 a roof yields two barrels for each space ten feet square, 

 and seventy-two barrels a year are yielded by three 

 feet of rain. A barn thirty by forty feet supplies annu- 

 ally from its roof 864 barrels, or enough for more than 

 two barrels a day for every day in the year. Many 

 farmers have in all five times this amount of roof, or 

 enough for twelve barrels a day yearly. If, however, 



