Portion of a horizontal force magnetograph, consisting of a rectangular bar magnet mounted in a carrier and fitted with two mirrors. The upper mirror, which is clamped at three points on the periphery, is carried in a heavy frame that is orientated by means of a tangent screw. The framework also carries a magnet clamp, below which the second mirror is fixed.

This was used at the Royal Observatory from 1840 for indicating variations in the Earth's horizontal force. Read hourly by eye for 7 years, until photographic self-registration was introduced. It was replaced in 1914 with a smaller magnet.