BRIGADIER M. N. MacLEOD delivered the Thomas Young Oration of the Pysical Society on March 24, taking as his subject “Some Recent Developments in British Surveying Instruments”. Referring to the aerial camera as a surveying instrument, he pointed out that all parts of the ground must be photographed from two separate points. Plotting is done by combining, stereoscopically or otherwise, two such pictures. Plotting from tilted photographs requires complicated apparatus. In the first plotting machines the air positions of the camera and the direction of its optical axis had to be determined by a “resection in space” from control points on each photograph. There are theoretical as well as practical objections, however, to such independent determinations. In 1926, Dr. Fourcade propounded the idea of first determining the relative positions of the air cameras without reference to ground points, and devised a stereo-goniometer for this purpose. This principle has been made the basis of a new British automatic plotting machine constructed for the War Office and about to be delivered to the Ordnance Survey. In the majority of automatic plotters, the photographic picture is reconstructed stereoscopically and the image of a mark in the stereoscope is superimposed optically on the view. The visual or ‘floating’ mark can be moved about by the plotting mechanism and can be made to coincide in depth as well as in alignment with any point in the view. The apparent movements of the mark are transmitted mechanically to a plotting pencil. In all automatic plotters, however, much time is taken in the preliminary adjustment for reconstructing the stereoscopic view and determining its scale and orientation. It is hoped that the new instrument will enable this preliminary setting to be done decidedly more quickly than is the case in any existing foreign model.