Notes on [Thomas] Maclear "base" work to measure the Arc of Meridian, by Sir John Frederick William Herschel Note in Sir John Frederick William Herschel's hand summarising the work conducted by Thomas Maclear in South Africa for a baseline to measure the Arc of Meridian and correct the previous arc calculated by Abbé Lacaille. Includes the triangulation method with landmarks of reference, specifically for the Zwartland Plain, [Swartland] and Saldanha Bay and Schapen Island [Schaapeneiland]; also includes the list of people "at work": [Thomas] Maclear; Charles [Piazzi-]Smyth; Captain [Alexander] Henderson; Lieutenant Cust?; [William] Mann noted 'occasionally'. 1840s