The method of taking high-altitude observations at sea described by A. N. Black in the April number of the Journal is not widely known and is undoubtedly of practical value, though it is not new. In his article Mr. Black regretted that he had had no opportunity of testing the method under practical conditions as his cruising range did not extend into the tropics. For this reason the results of some actual observations taken by the present writer some twenty-five years ago using this method may be of interest to Mr. Black and to readers of the Journal.

The author is in entire agreement with the first ten lines of Mr. Black's paper, though the term slant altitude or inclined altitude might express better what is being measured than horizon distance, which may be misleading. He does not however agree that the method is sound in theory for ‘normal’ altitudes, as will be explained. The method is in fact both practically and theoretically restricted to use with zenith distances of about i°, which is the limit actually considered in Mr. Black's paper.