THE FARMER'S MAGAZINE. 



375 



whole pressure is concentrated, of water against a per- 

 pendicular dam is at one-third of the height from the 

 bottom. 



Water at the temperature a little above the freezing 

 point expands, and this with so much force as to burst 

 the strongest pipes, or the toughest and hardest rocks, 

 if so confined as to offer resistance to the expansion. 

 The fact of the expansion of water in freezing is obvious 

 from the fact that ice always floats and sustains a con- 

 siderable burthen upon water. 



Water has a constant tendency to descend ; but the 

 instant it meets with resistance it exerts its force in 

 every other direction. 



The above-mentioned force in water is invariably 

 exerted until it has found a level, and it can then only 

 be said to be at rest ; and it will, therefore, continue to 

 rise from a lower to a higher level until the two surfaces 

 at each end in a bent tube are at the same height ; and 

 so in a porous stratum filled or covered by an imper- 

 meable one ; and whenever this equilibrium is attained 

 it remains in that state until disturbed. 



Water ascends through the soil by capillary attraction, 

 wherever the lower portion of the soil rests in water. 



Such, then, being the properties of water, at least so far 

 as they relate to the subject under consideration, the 

 different systems of draining previously enumerated may 

 be discussed. 



Trunk Draining frequently involves works of con- 

 siderable magnitude and complication, which will re- 

 quire the professional assistance of an engineer, whose 

 attention has been directed to liydraulic phenomena, 

 in designing them, and under whose direction and in- 

 spection the works will require to be carried on by con- 

 tractors accustomed to execute such operations. 



Although very extensive works in the kind of draining 

 now referred to may require the direction of the pro- 

 fessional engineer, yet it will seldom occur that land 

 draining will not require recourse being had to the 

 formation and improvement of open water-course on a 

 minor scale, and within the competence of an intelligent 

 farm overseer to direct the execution, in which there are 

 certain maxims, as regards the motion and effluence of 

 water, necessary to be observed with a view to efhcient 

 drainage, which are seldom attended to in the designing 

 and laying out of such works by unprofessional persons. 



The following are the results of observations by an 

 eminent engineer of the past age — Smeaton : "That, 

 mathematically speaking, if a river runs upon a bot- 

 tom lying with an equal descent, the velocity of the 

 water will be the greater the further it runs. Thus, 

 supposing the descent to be one foot per mile, the water 

 will have acquired a velocity of eight feet per second. 

 After four miles, its velocity would be sixteen feet per 

 second ; and, at the end of six miles, it would be at the 

 rate of thirty-two feet per second ; its velocity at every 

 point being the same as would be acquired by a body 

 falling the same perpendicular height. 



"These rules, with respect to the motions of rivers, 

 are very far from being observed, on account of the 

 perpetual obstruction that the water meets with against 

 the eidet and bottom, which countersct the power of 



gravity,' and reduce the water to a uniform motion 

 where the declivity of the bottom and sides are regular. 



"That, supposing a given quantity of water to be 

 carried oflf, the smaller the descent, the width and depth 

 or section of the river must be greater ; for the water, 

 running in a larger body, and slower, meets with a less 

 proportionable obstruction from the sides and bottom. 



" That also, supposing the same quantity to be dis- 

 charged, the larger the body it runs in, and the slower 

 the motion, the more liable its course is to be ob- 

 structed by mud, weeds, &c., &c. 



" That large and deep rivers run sufficiently swift, 

 and discharge vast quantities of water, with a descent of 

 one foot per mile. 



"That small rivers and large burns require about two 

 feet per mile. 



" That small burns hardly keep an open course under 

 four feet per mile. 



" That ditches, covered drains, &c., require at least 

 eight feet per mile. 



" That when a tract of ground lies level, it is much 

 preferable (where it can be done) to lay the drain deep 

 at one extremity, to give a greater descent upon the 

 bottom all the way, than to make abroad and less deep 

 drain all the way upon a less descent, for the former 

 method is attended with less extent of digging, and 

 better calculated to keep an open course." 



Although the descents mentioned above are the least 

 required, under the several circumstances, for water to 

 have a sufficient velocity to keep a clear course, yet the 

 velocity of the current must be adjusted in equilibrio 

 with the tenacity of the channel. In respect to the 

 latter condition in the adjustment of the velocity of 

 water-courses, it has been determined by Du Buat and 

 others, that the following velocities per second at bottom 

 will move — 



Velocity 

 in inches. 



CUiy ■ 3 



Fiue 3ind. . ,. .. .. .. ..6 



Coarse saud . . . . . . . . . . 8 



Water- worn gravel an inch in diameter. . .. 12 



Angular atoues the size of a hen's egg. . . . 36 



In open water-courses the velocity is greatest at the 

 surface, and in the middle of the stream, from which it 

 diminishes towards the bottom and sides, where it is 

 least. 



The velocity at the surface of a stream is easily deter- 

 mined by the distance a floating body is carried in a 

 given time, from which the velocity at the bottom may 

 be found by subtracting 1 from the square-root of the 

 velocity at the surface, and the square of the remainder 

 is the velocity at the bottom. The mean velocity is half 

 the sum of the velocities at the surface and the bottom. 



An element in calculating the velocity of water- 

 courses is what is called, by some writers on hydraulics, 

 the hydraulic depth, and by others the radius of the 

 section. It is the quotient arising from the area of the 

 section being divided by the bounding perimeter of the 

 channel. Thus, in a semi-circular channel it is the 

 area of the semi-circle divided by the serai-circumference 

 of the whole circle ; or, in case of a water-course being 

 a figure bounded by straight lines, it is the quotient of 



