THE issues of the Journal of the Franklin Institute of January and February 1944 contain an article by C. D. Tuska entitled "Historical Notes on the Determination of Distance by Timed Radio Waves". This article traces the historical development of methods of measuring distance by means of radio waves using both the frequency modulation and amplitude modulation or pulse methods. It is now about twenty years since the classical experiments of Appleton and Barnett provided a direct measurement of the height of the reflecting layer in the ionosphere using the frequency variation method; while, shortly afterwards, Breit and Ture demonstrated the use of short trains of waves of about one millisecond in length for making the same type of measurement. Many variations and improvements on these two methods have been made from time to time by those engaged in studying the properties of the ionosphere as a medium for reflecting radio waves back to the earth's surface. Methods based on the same principles have also been used for determining the altitude of aeroplanes by reflecting waves from the ground beneath. The article referred to above describes briefly the various methods which have been devised to meet these applications by the aid of a review of the published scientific literature, and especially of the publications of the United States Patent Office for the past ten years or so. As the author states, the notes are necessarily incomplete for the war period, on account of the scarcity of publications for security reasons. The bibliography of eighty-one references appended to the article may, however, be useful to those concerned with tracing the historical development of a comparatively modern application of radio science.