When the writer undertook the preparation of this paper, he rapidly discovered that it was not easy to find any branch of the subject which had not already been dealt with exhaustively by far abler pens than his. One or two matters have, however, been chosen about which information is somewhat meagre or scattered, and which may not, perhaps, be familiar to the aircraft engineer.

First a review will be made of the various types of electronic engine indicator which have been developed up to the present time; this instrument is coming into prominence now, and although it has by no means reached a state of perfection, it is a valuable asset in the hands of the research engineer for the study of fuel problems and other minor points.