As far as the writer is aware, Mr. Horace Darwin was the first to call attention to the fact that there is an essential difference in action between a spring-controlled and a gravity-controlled air speed indicator on an aeroplane.

This fact was put forward in the Wilbur Wright lecture of last year and the conclusion then reached was definitely in favour of the use of the gravity type, as being more useful to a pilot on a steeply banked turn.

Since that date, however, the discovery has been made that this type fails to give warning of “ stalling ” in straight flight (the very purpose for which the instrument is installed) and equally fails under the conditions of a banked turn.

The writer believes that he has the priority in this discovery, and therefore takes the present opportunity of publishing the following formal proof of the scientific theorem involved.