Item 226 - Letter from Peter Barlow

4 pp.

PB explains his comments concerning the effect of iron on a ship's compass. His remark on page 79 was made on the ground of theory and not from actual experiment on ship board [Essay on Magnetic Attractions, 1820?]: 'you will have seen that the distance of the centre of attraction of the iron (speaking at present of balls only) and the relative position of the compass and needle as referred to an imaginary sphere which may be supposed to circumscribe either the ball of iron or the compass, its poles in either case corresponding in position with the magnetic poles of the dipping needle at the place of observation'. PB then provides an example accompanied by diagram and explanation. With regard to the 'accuracy that has been obtained - I have no better authorities on these subjects but what are contained in my supplement' [Appendix Containing an Account of the Experiments Made on Board HMS Lever, Conway and Griper, for Correcting the Local Attraction of these Vessels, 1824?].