HC Deb 16 July 1912 vol 41 c194

asked the Secretary of State for War (16) whether in the official estimate that at a height of 5,000 feet in an aeroplane one can get a vision over 10,000 square miles he included normal weather; if so, will he inquire from the meteorological experts upon how many days during the last twelve months it would have been possible in the North Sea to see the distance indicated; (17) whether in the official estimate that at a normal height of 4,000 feet to 6,000 feet an aeroplane can obtain a vision over 10,000 square miles he based his calculations upon actual practical experiment or upon theoretical calculation of the earth's surface; if the former, upon what dates and by whom were the experiments made, and how many days in the year is it estimated that the meteorological conditions will be such as to allow approaching ships to be seen at such a distance; and what warning in point of time would an aeroplane be able to give of a raid timed to arrive at dawn after a dark night?

Colonel SEELY

The distance of the visible horizon at sea on a clear day from an observer at a height of 5,000 feet, allowing for normal refraction at midday, is, I am informed, 93½ miles; the area under observation would thus be approximately 27,500 square miles.


May I ask if, as the right hon. Gentleman is going on theory only, if the statement made by him the other day that you could see 10,000 square miles, does not, if you adopt his formula, only include half of the horizon, and is it correct to include the question of refraction, which ought surely to be discounted on the basis of his calculation?

Colonel SEELY

I have given what I think is a very full reply. I have referred to the experts at the Admiralty and elsewhere in order to verify the figures, and I think the Noble Lord will find them accurate.