The periods during which the conditions would be favourable for our enterprise depended upon various factors. The extent to which we could make use of any particular date during one of those periods depended, in turn, upon meteorological conditions. It has been shown elsewhere that, for the purpose of utilising the artificial fog, we required a wind blowing more or less toward the Belgian coast from seaward. It was also necessary for the wind to be light so that the small craft would not be hampered by rough seas. Light winds are often accompanied by fog, especially in the North Sea, and fog would be a serious obstacle. Rain would be detrimental to the use of aircraft. The depths in the entrances to the canals at Zeebrugge and Ostende were such that the blockships could only navigate during the period around high water. It was, therefore, necessary for the vessels to arrive at about that state of the tide. Again, it was essential to carry out the operation during the night for reasons already mentioned. In this respect, however, the word operation must not be used too loosely; the periods of approach to the objectives and of retirement therefrom must both be included in that term. The approach had to be {132} undiscovered till the latest possible moment. That necessitated darkness throughout the approach, which latter may be considered as comprising the passage during the last twenty miles of the oversea voyage. Obviously the greater part of the whole passage conducted during darkness the less would be the chance of losing the element of surprise. Likewise the retirement must be made before daylight if the concentration of the shore batteries was to be avoided. I have already stated that the German guns could make things very uncomfortable for ships up to a maximum distance of fifteen to twenty miles, provided that the ships could be seen. The retiring forces should, therefore, be outside that range before there was sufficient daylight to see so far; the latter state of visibility would obtain at least half an hour before sunrise. Allowing a speed of about ten knots for retirement, this meant that the ships must leave the coast about two and a half hours before sunrise after completing the operation. The attack itself was expected to continue for about one and a half hours. The reader does not require to be an advanced mathematician to realise that the attack must commence not later than about four hours before sunrise, that is, during the middle portion of the night. Of course, the attack was the all-important matter; any question of safe retirement must be a secondary consideration. Nevertheless, the question of retirement, similarly to that of rescuing the blockships' crews, had to be taken into account. [Illustration: Specimen Diagram for ascertaining Available Period. (Transcriber's note: this diagram occupied page 133 of the source book)]