Copy of a letter from Charles Wright Younghusband, Woolwich to Maclear Younghusband recommends using the deflecting magnet D. 3. 3. 67 with the unifilar for observations ‘largely’ for absolute horizontal intensity. Reference to determining the values of P and 2, and discussion regarding their coefficients. Discussion regarding the moments of inertia and the temperature coefficient of magnet D. 3. Reference to a ‘large telescope with a detached scale’. 31 March 1851