SUMMARYA new finite differenoe scheme for the convection-diffusion equation with variable coefficients is proposed. The difference scheme is defined on a single square cell of size 2h over a 9-point stencil and has a truncation error of order h4. The resulting system of equations can be solved by iterative methods. Numerical results of some test problems are given.

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A high-order finite-difference approximation is proposed for numerical solution of linear or quasilinear elliptic differential equations. l h e approximation is defined o n a square mesh stencil using nine node points and has a truncation error of order 11'. Several test problems. including one modeling convection-dominated flows. arc solved using this and existing methods. The results clearly exhibit the superiority of the new approximation, in terms of both accuracy and computational efficiency.

In this article, we give a simple method for deriving finite difference schemes on a uniform cubic grid. We consider a general, three-dimensional, second-order, linear, elliptic partial differential equation with variable coefficients. We derive two simple fourthorder schemes. When the coefficients of the second-order mixed derivatives are equal to zero, the fourth-order scheme requires only 19 grid points. When the coefficients of the mixed derivatives are not equal to zero and the coefficients of U,, U,, and U, are equal, we require the 27 points of the cubic grid. Numerical examples are given to demonstrate the performance of the two schemes derived. There does not exist a fourthorder scheme involving 27 grid points for the general case.

SUMMARYThe number of flying insects in the sheltered zone to leeward of artificial windbreaks of 0, 25, 45 and 70% permeability was greater than in unsheltered areas. Insects accumulated in the air nearer to dense windbreaks than to more permeable ones, and the position of maximum aerial density of insects coincided with that of maximum shelter. The more dense the windbreak the greater the numbers in the air to leeward. Absolute wind speed slightly affected the position of maximum aerial density behind a 45% permeable fence.