THERE are in the Science Museum three examples of the set-square to which Mr. Smeal refers. They have been here since 1876, in which year they were lent by the inventor, Edward Bing, a member of the staff of the Waggon Works at Riga, for exhibition in the Special Loan Collection of Scientific Apparatus. There is a short description in the third edition of the catalogue of that collection, published in 1876, at p. 14, and I have no record of any earlier published description. One of the examples is of steel and the other two of wood, the hypotenuse in each case being about 7 in. long. The inventor's MS. label inside the mahogany case containing the steel set-square reads:—“Bing's Circular Square Kreiswinkel Equerre circulaire Cosinus α = π/4. (α = 27° 35′ 49.636″).“