561,106. Measuring angles. LEE, W. Aug. 2, 1942, No. 5949. [Class 97 (ii)) An apparatus for determining or calculating heights applicable to the observations, taken at observation posts, of the flight of aircraft comprises two or more spot lights adapted to be adjusted and oriented to the relative positions and bearings of corresponding observation posts and also to be oriented and adjusted to bearings and elevations reported from such posts, a screen on to which such spot lights are projected or reflected, means for adjusting the screen to orient the light spots and means for indicating the height corresponding to any adjusted position of the screen. Figs. 1 and 2 show an apparatus using two spot light units A, B mounted on a base 20 such that unit A is fixed in position while unit B is slidable on guides 21 to represent to scale the distance between the represented observation posts. Each unit comprises a pivot 25 on which are mounted for rotation a compass dial 26, divided into 60 divisions termed minutes, and a platform 27 having an indication arrow 28. On each platform 26 is mounted a post 29 carrying an arm 30 rotatable about its axis and bearing a spot light provided in a casing 31 having an aperture 34 througn which a beam or pencil of light projects. Mounted also on 'the platform 26 of the unit A is a post 39, having a scale representing heights, and mounted on such post for vertical adjustment is a slide 43 carrying a screen 44 on to which the spot lights are projected. Also on the slide 43 a rotatable compass disc 49 and pointer 50 is mounted for adjustment along guides. The units A, B are provided with centre marks 24 and the screen 44 is provided with a scale 45, 46 representing distances in miles. In the example specified, the observation posts are nine miles apart and the unit B is accordingly moved to such a position represented on the'scale 21 on the base 20. The bearing of the represented post B to that of A is 07 minutes and the compass discs 27 are rotated so that such minutes are in line with the marks 24. The observation sent in from represented post A is bearing 04 minutes and height 8000 feet. The platform 26 of unit A is accordingly rotated so that the indicator 28 points to 04 minutes on the compass disc 27 carrying around with it the post 29 for the spot light and the screen 44. The spot light is angularly elevated to correspond with a reading of 8000 feet on a dial co-operating with the arm 30 of the spot light. Similarly, compass disc df unit B is rotated to indicate 07 minutes against the mark 24 and the platform 26 and spot light are operated to correspond to the observations sent in from the represented post B, viz: 46 minutes and height 29000 feet. The compass disc 49 is set at 04 minutes and the pointer 50 at 22¢ minutes, the latter representing the S.E. direction which represented post A reports the aircraft to be flying. The slide 43 with its screen 44 is now adjusted vertically on the post 39 until the pencils of light indicated by lines 52, 53 fall on the screen at spots 54, 55 on a line 56 which is parallel to 'the reported direction of flight indicated by the pointer 50 on the compass disc 49. The height at which the aircraft would be shown on the scale on the height post 39, viz : 15000 feet and its distance from represented post A would be 8¢ miles. If there is a time lag between the observations and the aircraft in the meanwhile has changed course or height, the spot 'lights the screen cannot be oriented on the indicated time of flight and fresh observations must be taken. In a modification, three spot light units representing three observation posts are used and the apparatus uses a height screen which is vertical and is adjustable horizontally. The height of the aircraft and its distance are obtainable in a manner similar to that employed with the apparatus having two units. The course of the aircraft will be indicated by the line on which the three spots of light fall. If they do not fall on a'straight line fresh observations must be taken.