I’ve written quite a bit about signals intelligence (SIGINT) and attempts to block that gathering in the context of the war at Charleston. Stretching the definition a bit, we might also consider Emissions Intelligence (EMINT) in context of the blockade operations (and I’m probably bending the modern definition a bit, as EMINT specifically involves collecting sensor and other forms of electronic transmissions… but in the Civil War sense, can we include “lights”?). During operations the blockaders used running lights, signal lights or flares, and similar means of passing messages visually. The meaning of these was, as one might expect, something the Confederates sought to understand. So the Federals adopted some rather complex systems to keep the Confederates out of the loop. On September 23, 1864, Rear-Admiral John Dahlgren issued a set of instructions detailing the use of ships lights and signals at Charleston. Long read, but allow me to submit it into the “register” here for reference:
The following running lights, specified for each day of the month, are to be hoisted when in chase or approaching the limits of the blockade after an absence from it.
When in chase, they will be shown occasionally for two or three minutes at a time, after the established signal for an attempt to run the blockade inward or outward has been made, to indicate the direction of the chase.
When returning to the limits of the blockade after an absence from it, the running lights are to be shown until the ship’s number or signal of recognition has been exchanged with one of the vessels on the blockade.
A vessel discovering and desirous of ascertaining the character of another at night will show a light (signal lantern) of the same color as the upper light of the running lights designated for the night until answered by the vessel to which it is shown by a light corresponding with the lower light of the running lights designated for the night. Then the vessel which made the signal first will obscure her light, which is to be followed immediately by the vessel to which the signal was made obscuring hers. The latter vessel will then flash twice with a light of a different color from that shown by her in answering, with an interval of five seconds between the flashes, which must be answered by the vessel which made the first signal by two flashes made in the same manner, with a light of a different color. The signal lights should be held, if practicable, in a position so that they may be seen only by the vessel to which the signal is made.
Got that? To ensure no misunderstandings, Dahlgren offered some examples of this pattern:
Example, 1st day of the month:
A discovers B.
A shows a white light.
B answers with a red light.
B flashes twice with a white light.
A answers by flashing twice with red light.
Dahlgren went on to provide two more examples before relating general instructions about security of the light patterns.
A knowledge of the running lights is to be confined to commanding, executive, and signal officers. Officers in charge of the deck should only be informed of the running lights for the night at sunset, daily. It is also desirable that a knowledge of the signal to ascertain character should be confined to officers alone, who should give the necessary directions to make them to quartermasters or persons detailed for that duty.
In the event of there being a doubt about the character of a vessel after the forgoing signals have been exchanged and repeated, fog signals of ship’s numbers may be exchanged.
And the instructions turned to fog signals next:
Steamers will make fog signals with their whistles; sailing ships with their bells.
Then Dahlgren turned to “Special signals regarding rebel movement”:
Blockade runner going inward: Rocket and red light (Coston). Going outward: Rocket and white light. The vessel that discovers the stranger will fire on her while signaling.
This will be understood as a signal for the whole squadron to be on the alert; but no vessel will leave her particular station to chase unless she discovers the blockade runner or can steer a course understandingly to cut her off.
Rebel rams in sight and near: Rocket, red, white, red. When the signal is made, a single red light at the masthead of the vessel commanded by the senior officer present will indicate that all the heavy-built and armed vessels are to assemble near that vessel.
The light-built and armed vessels will be on the alert to discover and capture any vessels attempting to run the blockade, and render such other services as the occasion may suggest to their respective commanders.
Torpedo boats in sight and near: Rocket, white, red, green.
Enemy’s boats in sight: Green.
Vessel in danger from fire of another: Running lights.
Signals for “Assistance required”:
Show in a horizontal position, about 10 or 15 feet distant from each other, a red and white signal light (lantern) to be screened or held in a position, if possible, that it may not be read by the enemy; particularly if the vessel making the signal should be ashore or aground. This signal is to be answered by Coston’s “Answering.” [Red-White-Red flare]
Continuous and rapid firing of guns will be understood as a signal that assistance may be or is required by the vessel firing the guns, and the vessel nearest to the locality of firing will proceed to give assistance, but will not be absent from her station longer than is absolutely necessary.
The “Answering” (Coston) followed by “Preparatory” (Coston) will indicate that all of the vessels employed on the blockade are to repair to the locality where the signal is made.
Two perpendicular red lights (signal lanterns) shown at the masthead or yardarm will annul the signal last made
Strange vessels approaching the blockade in the daytime:
In the event of a strange vessel approaching the limits of the blockade during the daytime, as a general rule, the blockading vessel nearest to her will ascertain her character, etc., by boarding or speaking, and report to the senior officer present, by signal or otherwise. In communicating with a strange vessel, a blank cartridge is first to be fired, if necessary, as a summons to heave to, and a shot is not to be fired unless the summons by blank cartridge is unheeded. Vessels of doubtful character and foreign men-of-war are to be accompanied by the vessel communicating with them to, and outside of, the anchorage of the senior officer present.
Commanders of vessels composing the blockade are expected to observe and exact from officers under their respective commands the greatest vigilance, attention to, and promptness in making and answering signals, and to use the utmost care in not firing into a vessel until well satisfied she is attempting to violate the blockade.
As for other lights which might displayed:
No lights will be shown by the blockading vessels at night excepting for signal purposes.
All signals conflicting with the above are revoked.
A little lengthy, but these instructions provide another insight into the operations along the coast of South Carolina in the maintenance of the blockade.
(Citation from ORN Series I, Volume 15, pages 685-8.)