HOW OFTEN does one hear the aggrieved tones of a sufferer asking why he must put up with leakage troubles ? All too frequently such situations occur in all of the very many branches of hydraulics and it seems a pity that in many instances the trouble need not have occurred at all. Obviously, the art of scaling is far from perfect and, in fact, this branch of technology is undergoing some fairly radical development at the present time. Nevertheless, although seals are a necessary evil, many of the occasions when seal trouble is experienced could have been averted if the correct seal had been chosen in the first place and then used to the best advantage.