Many types of detonation may be distinguished, and the causes are to be found in the engines as well as in the fuels. Obviously one single cause would not be expected to be found, but, on the contrary, quite a number of properties which will be more or less of influence under different engine conditions. These may differ widely, yet there is more unity in the multitude of causes of detonation than might be expected, and this the authors will endeavour to illustrate. They must apologise for mentioning many well-known, in addition to new, views, but it is necessary to do this for the general exposition of the material.

Two principal types of engines should be considered to-day :–

1. (1) That in which the fuel is added to the air at the end of compression (in air compressing or Diesel engines), and

2. (2) That in which the fuel is added to the air before compression (mixture compressing, e.g., petrol engines).

Most methods of increasing the output of petrol engines seem to attract the “ demon of knocking ” ; the reverse is true for Diesel engines, which is hopeful for their future development.

Referring to Fig. 1, let xOy be the plane of rotation of a rigid rod AOA' consisting of a series of pairs of masses symmetrically disposed about O. Thus at a distance r on either side of O are masses mr. Let AOA' rotate with uniform angular velocity Ω, about the axis Oz and let the plane xOy simultaneously rotate about the axis Ox, supposed fixed in space, with constant angular velocity w. Let ρ be the perpendicular distance mrMr of mr to the axis Ox.

Many “figures of merit” for the comparison of aircraft have been advanced at various times, but in those cases where the basis of comparison has been logical no consideration has been given to first cost, while in those cases where money cost is included, serious objections can be raised to the value of the comparison made.