Some time ago a committee at the Ministry of Labour asked me to lecture on the above. Procrastination, ill‐health, a good deal of work and other causes resulted in my addressing my audience with apologies and a few scrappy notes. The meeting was kinder that I deserved, but the following is a digest not so much of what I said, and is certainly not in the disorder in which I said it, as what I should have said if I had been able to give some time to its preparatio