Note 69 — PART SECOND. NOTES ON THE ILLUMINATION OF LIGHTHOUSES, WITH SHORT NOTICES OF THEIR EARLY HISTORY. (continued 2)
Reading edition
The angle OAX is easily found, as will be seen by referring to fig. 73, p. 277; for, AH being horizontal by construction and AO vertical, HAO = 90°; and HAC and CAU being both known, we have
OAX = 90° - (HAU + UAX) = 90° - (HAU + ¹⁄₂ CAU).