D Y K 



Mere the first vertical column contains the several months in the year, 

 and that part of the other columns immediately opposite contains the do- 

 minical letters : the under part contains the days of the month on which 

 the Sundays happen ; and hence the other days of the week are easily 

 found. 



DOUBLE Stars. See Stars, 



DYKE. (Bland.) 



A mound or obstacle opposed to the effort made by a fluid to spread 

 itself. 



1. Let A B C D be a vertical section 

 of a dyke opposed to the stagnant fluid. 

 Its parts are supposed to be so con- 

 nected, as to yield to the pressure of 

 the fluid, either by turning altogether 

 round the point A, or by sliding along 

 the horizontal base D A. 



2. Supposing the dyke to yield by turning round A, to determine when 

 there will be an equilibrium. 



Let H K x t E K y, E F a, AD = b, F D = c, s = specific gra- 

 vity of fluid, *' = do. of dyke, Q the product of the area A B C D mul- 

 tiplied by the distance of A from the vertical passing through the centre 

 of gravity of the area ; then in the case of equilibrium, 



% so? = s' Q 4. s. fl. (b c -j- a:), ydx. 



3. Supposing the dyke to yield by sliding along its horizontal base : to 

 determine when there will be an equilibrium ; neglecting the vertical 

 pressure of the fluid. 



The base being horizontal, the mass which it sustains is supported 

 against the horizontal force of the fluid only by its adhesion to the base, 

 and the resistance arising from friction. Supposing these resistances 

 n times the weight of the dyke (n being determined by experiment) j 

 and let P the area of the section A B C D j then 



4. If A B and C D be straight lines, i. e. if the sides of the dyke be rec- 

 tilinear, and A D, B C horizontal ; to determine the equation of equili- 

 brium of Art. 2. 



LetCM = ft, MD = e, A N = ', then 

 83 



+ f ~^. a 



