The problem of the equivalence of binary forms is of great importance, both historically and intrinsically, and is also significant for the problems of the so-called canonical and automorphic forms. It consists in deciding whether two given forms are equivalent, i.e. transformable one into the other linearly, and, when they are, in finding all the linear substitutions transforming one form into the other.

A collineation in a space of paths is defined as a point transformation which carries paths into paths. Such transformations were first studied by L. P. Eisenhart and M. S. Knebelman. Subsequently J. Douglas introduced the geometry of K-spreads and E. T. Davies has shown that the results for an affine space of paths can be extended to these more general spaces and written in a very elegant form by the aid of Lie derivation.

1. I. N. Sneddon has recently considered(11) the two-dimensional problem of the stress distribution due to a force uniformly distributed along a strip in the interior of a semi-infinite elastic medium by integrating the distribution due to an isolated force. The method of solution for the isolated force problem differs from that originally used by Melan(9), but both writers employ real variables. A solution of this problem, using complex variables, has been given by Stevenson and a considerable saving of labour is revealed. A complex variable method of solution for problems of stress distributions in an elastic medium bounded by a plane face was, however, given earlier by Sen(10), although he only made applications to problems in which the force system was applied to the plane face. Sen's method is somewhat different from that used by Stevenson and gives results more directly for many problems, including the problem considered by Sneddon.

1. The reflexion of waves on the surface of water by a thin plane vertical barrier is considered and the coefficient of reflexion (the ratio of the amplitudes, at a great distance from the barrier, of the reflected and incident waves) is calculated. If the top edge is at a depth a below the surface, it is found that the coefficient of reflexion is about ¼ when where T is the period of the incident waves, so that the condition that the coefficient may exceed ¼ is a .

1. In a recent paper(1) the partial differential equation governing the symmetrical vibrations of a thin elastic plate was reduced to an ordinary differential equation by the use of the Hankel transform method. By the discussion of the solutions of the latter equation and by the use of the Hankel inversion theorem an account was given of the free and forced vibrations of the plate under symmetrical conditions.