The primary object of flying is movement over the surface of the earth in the shortest possible time, and thus it comes about that the performance of an aeroplane is that characteristic which commands greater attention than all others.

This method has been designed to give all necessary performance data, concerned with the take-off and landing of aircraft, in a cheap and efficient manner. The complete path of the machine from the start of the take-off run to beyond the screen is recorded on a single photograph and from it the reduction of the data is done readily and simply. In the case of a take-off the data obtainable is as follows :

1. (a) Take-off run.

2. (b) Nature and length of the transition.

3. (c) Height at any screen.

4. (d) Distance, velocity and acceleration at any point from the start.

5. (e) Angle of climb.

6. (f) Rate of climb.

7. (g) Any irregularities in the flight path.

Similar data can be obtained in the case of a landing. Any information may be extracted independently and the photographic negative filed for future reference.

The camera used is simple and inexpensive, both in construction and subsequent use. Four persons at present form the total ground staff and with an improved camera now under construction one of these is eliminated.

The accuracy obtainable with the present apparatus is sufficient to fix the position of the aircraft to within ± 2 feet. Once the initial work has been carried out on any aerodrome, future use of this method has been found to be simple, quick and sufficiently accurate for all normal requirements.

After some general remarks upon the constant flight characteristics of birds belonging to any given species, the author describes his method of obtaining, approximately, a cinematographic reproduction of three geometrical projections of the wing-beat. Dealing with one particular case he determines the trajectory in space of a pigeon's wing tip during an entire beat, and, also, the speed and acceleration reached by it.

Reciprocal theorems are valuable devices in the attack on strength, stiffness and vibration problems in structures, often leading to elegant and simple solutions. Reciprocity is a fairly common property in various coefficients which arise in mathematical physics, as for example coefficients of mutual potential, capacity, induction, and so on. It is inevitably encountered both in theory and practice in almost any branch of physical science. So far as the stiffness of structures is concerned it seems that while Hooke is the mainspring of the theory of elasticity Rayleigh is the regulator of reciprocal relations in elastic systems.