Pre-radio technologies and early radio development

Prior to the introduction of radio, maritime communication was generally limited to line-of-sight visual signaling during clear weather, plus noise-makers such as bells and foghorns with only limited ranges. Beginning in the mid-1800s, an international convention was developed using special semaphore flags to exchange messages between merchant ships. In the same book, Examination Paper on the use of the International Code of 1901 it provided an overview of signaling proficiency that a candidate needed to master in order to qualify for a Certificate of Competency issued by the British Board of Trade Examinations. Over time an extensive vocabulary of signals was created, even as the expansion of radio was beginning to make visual signaling obsolete. The Urgent and Important Signals: Two Flag Signals section of Brown's Signaling reviewed over 600 basic signals, grouped by category, with meanings as diverse as "Where are you bound?" (SH), "In distress; want immediate assistance" (NC), and "Heave to or I will fire into you" (ID). And in addition to the two-flag signals, there were thousands of three- and four- flag groupings, for communicating a huge variety of messages, including ship identifiers, geographical names, temperature and barometer readings, compass points, and units of measurement. The development of radio resulted, by 1911, in the addition of two more visual signals -- ZMX for "Wireless telegraph apparatus" and ZMY for "Report me by wireless telegraphy" -- which heralded the beginning of a major decline in the use of seaboard visual signals. However, to this day NC continues to be an international distress signal when using flag signaling.
In the 1872 edition of the annual Journal reviewed the confusion and limitations often encountered, prior to the invention of radio, by ships trying communicate during emergencies, while suggesting that the "immediate object for the Telegraph Engineer... should be devising means for communicating at night, and in fog". Just a few years after Heinrich Hertz's historic proof of the existence of electromagnetic radiation, the Notes section of the April 10, 1891 The Electrician (London) included a strikingly advanced suggestion, that someday lightships might use microwave beams to overcome the problem of fog interfering with shore communication. In a December, 1891 lecture given at Inverness, Scotland, Frederick T. Trouton returned to this topic, noting that "There is little doubt that a powerful beam of this sort would, unlike light, be unabsorbed by fog; so, looking into the future, one sees along our coasts the light-houses giving way to the electric house, where electric rays are generated and sent out, to be received by suitable apparatus on the passing ships, with the incomparable advantage that at the most critical time--in foggy weather--the ship would continue to receive the guiding rays." A similar prediction appeared in the July, 1892 issue of The New England Magazine, as an extract from Elihu Thompson's Future Electrical Development stated "electricians are not without some hope that signaling or telegraphing for moderate distances without wires, and even through dense fog may be an accomplished fact soon", making possible a sort of radio-wave lighthouse.
Although it would turn out to take decades before practical microwave transmissions were developed, it was only a few years later that Marconi would introduce a successful system using longwave signals, and soon many of the larger passenger liners began carrying radio equipment. The addition of shipboard operators quickly captured the public imagination -- The Work of a Wireless Telegraph Man, by Winthrop Packard, from the February, 1904 The World's Work, recounted the activities of a Marconi operator on the passenger liner St. Paul, at a time when shipboard radio transmitters were so rare that operators had to wait for other similarly-equipped vessels to come into range.