TILE DRAINAGE. 113 



explained in geometry, and constantly used in trigonometry and 

 engineering. \ve gel the area of a cross-section of the inch tile, 

 and from this and the rate of flow we readily find the number of 

 gallons discharged each hour, which is about 862. Dividing the 

 whole number of gallons by this, and reducing, we find it would 

 take over 4 years and 11 months to discharge the rainfall of one 

 year, even if the tile works up to full capacity steadily all the 

 time. And even here the rate is overestimated, for none but a 

 very heavy grade will give a rate of four miles an hour in an inch 

 tile, owing to the great loss by friction in small rough tubes. 



Now, all such calculations are utterly unreliable, however 

 accurately made, unless they are based on careful experiment, 

 and take' into account all the variations of friction in tiles of 

 different caliber and inside smoothness, and varying pressure 

 caused by different grades, etc. They mislead in both directions. 

 On the one hand they assume that the tiles must discharge all 

 the water that falls on a field, whereas they discharge only the 

 surplus beyond the point of saturation, and sometimes for six or 

 eight months evaporation and absorption by the growth of 

 crops leave not a drop to reach the tiles. On the other hand, they 

 suppose a uniform daily or weekly rainfall, just up to, and never 

 beyond, the capacity of the tiles, whereas the tiles are sometimes 

 idle, as we have seen, for months, and sometimes, on a soil already 

 saturated, comes a rainfall of six inches in a single week, or even 

 three inches in a single shower. Now, the tiles must convey all 

 this away promptly, or it will wash and gully the surface-earth, 

 or stand stagnant for days. We must calculate, then, for the 

 maximum ever required of the tiles. To calculate according to 

 the annual rainfall is like calculating the annual traffic to be 

 borne up by a bridge, and from that estimating how strong it 

 must be each minute; whereas we know that, as with the " won- 

 derful one-hoss shay," so with the bridge, "the weakes' spot 

 must stan' the strain." even the heaviest strain that can ever 

 come upon it : and the bridge must be known, from formulas and 

 calculations based on experiment, to have as its " factor of safety" 

 at least live times the strength ever likely to be required of it. 

 The steel wire cables of the Brooklyn bridge have a strength of 

 80 tons to the square inch of section, and the four main cables are 

 each to be 16 inches in diameter, so that the aggregate strength 

 of the main span will be immensely beyond the combined force of 

 wind, storm, and burdens ever transported. 



These facts we all know. Now, it is the same way in drainage, 

 though the risks involved are not so enormous. The main drain 

 or drains must be up to the greatest emergency, or there is risk of 

 partial or perhaps total failure. What, then, are the greatest 

 emergencies? Facts alone can determine. Take a single one as 

 a sample. This year (1878) here, while wheat stood in the shock 

 we had over .'i inches of rain in 24 hours, on my farm. The ground 

 was fully saturated by previous rains, and there was but little 

 evaporation for a few days, and but little absorption by growth, 



