The annexed diagram is the dygogram Number I. for the ' Warrior,' 

 drawn accurately (by aid of a circular board rolling upon a fixed circular 

 board of equal diameter, in the manner described by Smith in the ' Ad- 

 miralty Manual,' Appendix II., under the heading " Mechanical Con- 

 struction of Dygogram No. I."), according to the following data deduced 

 from observations made at Spithead in October 1861. The notation 'H, 33, 



XH-1. 



23=-0-40S. 

 C = 0-090. 

 19 = +0-156. 

 (=+ 0-015. 



Mule for using Dygogram No. I. In the diagram Q is a fixed poiut of 

 the " Limagon," called " the pole of the dygogram." It lies in the 

 axis of symmetry, which is indicated by a dotted line. NQS, EQW 

 are two lines through Q, at right angles to one another ; and O, P 

 are two points, in positions fixed by the ship's magnetic elements. 

 The length OP represents " mean force on compass to north " (A.H). 

 Take any point B- on the curve, such that NOB/ is equal to the ship's 

 " magnetic course ;" then is EOP the " deviation " of the compass, 

 and OK represents the horizontal component of the force on it. 



