AND GAINING LAND FROM THE SEA, 



height from the bottom ; but it would be always more convenient 

 in practice to fix the side of the reservoir at the bottom, than to 

 allow the whole pressure to be supported by the prop, and it might 

 also be strengthened by means of fribs, thicker below than above, 

 so as to produce an equal strength throughout, wherever the prop 

 might be placed : but if the side were formed of a single plank, 

 of uniform thickness, the strain would be most equally divided bj 

 placing the prop very near the middle of its height. 



The strength of the materials employed for flood-gates and sluicei 

 requires to be determined according to the principles, which are 

 usually laid down, in treating of the passive strength of substance* 

 used for purposes simply mechanical ; but the calculations become 

 in this case much more intricate. Thus, if we have a circular plate 

 or plank, of a uniform elastic substance, constituting the bottom 

 of a pipe or cistern, and simply supported at the circumference, a 

 very complicated calculation is required for determining the pro- 

 portion of its strength to that of a square plate of the same 

 breadth, supported only at two opposite ends, since at each point 

 of the circular piece, there are two curvatures which require 

 to be considered. The square plate will support a column of 

 fluid twice as heavy as the weight which would break it, if placed 

 at its centre; and if the calculation be correct, a circular plate wilt 

 support a height of water nearly l6-7ths as great as a square plate. 

 But for ordinary purposes, it will be sufficient to consider the 

 strength as derived only from the resistance opposed to the flexure 

 in one direction, since the additional strength, obtained from the 

 lateral supports, may very properly be neglected, as only assisting 

 in affording that additional security which is always necessary, to 

 compensate for any accidental effects of the materials. It has been 

 asserted that the strength of a square plate is doubled when it i* 

 supported on both sides ; but this appears to be a mistake. 



We may, therefore, be contented with determining the strain ou 

 the materials in that direction fh which they afford the greatest 

 resistance, either from the shorter distance between the supports, 

 or by the disposition of the fibres; and it will be always most eli- 

 gible to combine these circumstances, so that the fibres of the wood 

 may be arranged in the direction of the shortest dimensions of the 

 sluice. If a sluice be supported above and below only, the greatest 

 tfrain will be at the distance of about 3-7ths of its height from 



