And, for the reduction of the base to the temperature of 62°, viz. for 8° on the brass scale, we have 27682,3944 -2,2497 feet 27680,1447 which, neglecting decimals, may be taken at 27680 feet. As to the probable error of the above conclusion, I know not how to form a just opinion. On ground sufficiently hard, and otherwise favourable, I think a base of 5 miles might be measured so accurately, as to afford a result not differing from the truth more than three inches: but, on this occasion, I should not suppose the error can be less than six, nor more than nine inches. Motives for adopting this supposition, have been related in a foregoing article. ART. XI. Calculation of the Sides of certain principal Triangles in Cornwall and Devonshire. Plate XXVII. Distance from Hensbarrow to St. Agnes Beacon, 97084,8 Feet. Phil. Trans. 1797. p. 461. Distance from Hensbarrow to Bodmin Down, 47337,2 Feet. Phil. Trans. 1797. p. 460. Mean distance from Hensbarrow to Trevose Head, 78096,4 feet. 81967,6 78093 Distance from Carraton Hill to Maker Heights, 82600,3 feet. Phil. Trans. 1797. p. 458. And, for the reduction of the base to the temperature of 62°, viz. for 8° on the brass scale, we have 27682,3944 -2,2497 feet 27680,1447 which, neglecting decimals, may be taken at 27680 feet. As to the probable error of the above conclusion, I know not how to form a just opinion. On ground sufficiently hard, and otherwise favourable, I think a base of 5 miles might be measured so accurately, as to afford a result not differing from the truth more than three inches: but, on this occasion, I should not suppose the error can be less than six, nor more than nine inches. Motives for adopting this supposition, have been related in a foregoing article. ART. XI. Calculation of the Sides of certain principal Triangles in Cornwall and Devonshire. Plate XXVII. Distance from Hensbarrow to St. Agnes Beacon, 97084,8 Feet. Phil. Trans. 1797. p. 461. Distance from Hensbarrow to Bodmin Down, 47337,2 Feet. Phil. Trans. 1797. p. 460. Mean distance from Hensbarrow to Trevose Head, 78096,4 feet. 11. Trevose Head Bodmin Down 42 33 52 -0,32 42 33 51,25 81967,6 78093 Distance from Carraton Hill to Maker Heights, 82600,3 feet. Phil. Trans. 1797. p. 458. Distance from Carraton Hill to Kit Hill, 33427 feet. Phil. Trans. 1797. p. 459. Mean distance from St. Stephen's Down to Carraton Hill, 52292,7 feet. 52994 52240,4 Kit Hill Black Down from {St. Stephen's Down 53128 62509,2 Hence the mean distance from Black Down to St. Stephen's Down, is 62508 feet. In the third triangle, the angle at Cadon Barrow is supplementary. When the observations were made at that station, a directionpost at Bodmin Down was mistaken for the staff, (to which it was similar in shape,) erected at no great distance from it. This error was not detected till long after: and, although it has been a maxim to which we have generally adhered, of observing all the angles of |