314 LECTURE XXVI. 



Iccted, as only assisting in affording that additional security which is always 

 necessary, to compensate for any accidental defects of the materials. It has 

 been asserted that the strength of a square plate is doubled when it is sup- 

 ported on both sides ; l)iit this appears to be a mistake. 



We may, therefore, be contented with determining the strain on the ma- 

 terials in that direction in 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 eligible to combine these circumstances, 

 so that the fibres of the wood may be arranged in the direction of the short- 

 est dimensions of the sluice. If a sluice be supported above and below only, 

 the greatest strain will be at the distance of about three sevenths of its height 

 from the bottom ; and it is at this point that the greatest strength is required. 

 But if the boards forming the sluice be fixed across it, in horizontal direc- 

 tions, their strength must be greatest at the bottom. (Plate XXI. Fig. 

 283.) 



In the construction of flood gates, the principles of carpentry must be ap- 

 plied in a manner nearly similar to that which serves for the determination of 

 the best forms of roofs. The flood gates, if they are double, without a solid 

 obstacle between them, must meet at an angle : and when this angle is very 

 open, the thrust against the walls or hinges must necessarily be very great. 

 If, however, the angle were too acute, the flood gates would require to be 

 lengthened, and in this case their strength would be far more diminished than 

 that of a roof similarly elevated, since the hydrostatic pressure acts always 

 with full force in a perpendicular direction. The thickness required for 

 each flood gate may be determined in the same manner as the thickness of a 

 sluice. 



Where a sluice board of considerable dimensions is to be occasionally 

 raised, it may be necessary to ascertain the force which will be required for 

 overcoming its friction; this friction is nearly proportional to the whole pres- 

 sure of the water, and may be found, with suthcient accuracy, in pounds, by 

 multiplying the square of the depth of the sluice, in feet, by 10. Thus, if 

 the depth be 3 feet, the friction or adhesion will be about SO pounds for each 

 foot of the breadth. 



