Scientific American Supplement, No. 711, August 17, 1889

The Governor of Gibraltar, General the Hon. Sir Arthur Hardinge, issued orders for the whole garrison to stand to their arms at dawn, and subsequent days, until the attack should be made; but by his express command no batteries were to be manned, or any troops moved from their alarm posts, until the signal was given that an attack was imminent. The alarm signal ordered was that of three guns fired in rapid succession from the Upper Signal Station on the summit of the Rock, to be followed, after a short pause, by two more shots. It was a matter of complete uncertainty as to the direction from which the attack would be made.