We shall also compare the linear theory with experiment; this is done in Section 5. The details of the experimental techniques and equipment are given in Section 4. We compare the reflection coefficient and the force coeffi cients, with those predicted by linear theory; various dis crepancies are noted, and an attempt is made to analyse them. Finally, we have also measured the mean horizontal forces on the cylinder; we compare these measurements with a simple formula obtained by Longuet-Higgins.7