Robert Hooke's Telegraph

List of images:
Construction of the Telegraph Apparatus
Synbols for letters of the Alphabet
Arrangement of lights for night communication

Dr. HOOK's Discourse to the Royal Society, May 21. 1684 shewing a Way how to communicate one's Mind at great Distances.

THAT which I now propound, is what I have some Years since discoursed of; but being then laid by, the great Siege of Vienna, the last Year, by the Turks, did again revive in my Memory; and that was a Method of discoursing at a Distance, not by Sound, but by Sight. I say, therefore, 'tis possible to convey Intelligence from any one high and eminent Place, to any other that lies in Sight of it, tho' 30 or 40 Miles distant in as short a Time almost as a Man can write what he would have sent, and as suddenly to receive an Answer, as he that receives it hath a Mind to return it, or can write it down in Paper. Nay, by the Help of three, four, or more of such eminent Places, visible to each other, lying next it in a streight line, 'tis possible to convey Intelligence, almost in a Moment, to twice, thrice, or more Times that Distance, with as great Certainty, as by Writing.

For the Performance of this, we must be beholden to a late Invention, which we do not find any of the Ancients knew; that is, the Eye must be assisted with Telescopes, of Lengths appropriated to the respective Distances, that whatever Characters are exposed at one Station, may be made plain and distinguishable at the other that respect it.

First, FOR the Stations; if they be far distant, it will be necessary that they should be high, and lie exposed to the Sky, that there be no higher Hill, or part of the Earth beyond them, that may hinder the Distinctness of the Characters which are to appear dark, the Sky beyond them appearing white: by which Means also, the thick and vaporous Air, near the Ground, will be passed over and avoided; for it many times happens that the Tops of Hills are very clear and conspicuous to each other, when as the whole interjacent Vale, or Country, lies drowned in a Fog. Next, because a much greater Distance and Space of Ground becomes visible, insomuch that I have been informed by such, who have been at the Top of some very high Mountains, as particularly at the Top of the Pike of Teneriff, that the IsLand of the Grand Canaries, which lies above 60 Miles distant, appears so clear, as if it were hard by; and I myself have often taken Notice of the great Difference there is between the appearing Distance of Objects seen from the Tops and Bottoms of pretty high Hills, the same Objects from the Top appearing nearer and clearer by half, and more than they do when viewed from lower Stations of the Hills; and this not only when the Space between them was Land, but where it was noThing but Sea. I have taken Notice also of the same Difference from the Prospect of Places from the Top of the Column at Fish-street-Hill, where the Eye is, in good Part, raised above the smoky Air below.

NEXT, the Height of the Stations is advantageous, upon the Account of the Refractions or Inflections of the Air; which Inflections of the Air are many and very great, sometimes in an Air which seems, to the naked Eye, the most clear and serene. Insomuch that That alone does wholly confound the Distinctness of Objects appearing at a Distance; now the greatest Part of those arise from Commotions of the more dense Air that is near the Surface of the earth, by the rarefactions of some Parts of it, caused by Heat; which rarefied Parts ascending, do make the Objects seen through it, to seem to dance and undulate, which is in great Part avoided, if the Prospect be from an higher Place. Besides, the Nature of the Air itself, at great Heights, approaches nearer to the Nature of the Aether, which more powerfully propogates the Impulses of Light.

NEXT, in chusing these Stations, Care must be taken, as near as may be, that there be no Hill that interposes between them, that is almost high enough to touch the visible ray; because in such Cases, the Refraction of the Air of that Hill will be very apt to disturb the clear Appearance of the Object, as I have often observ'd.

THE Stations being found convenient, the next Thing be consider'd, is, what Telescopes will be necessary for such Stations. And though 'tis true in all, that the longer the Telescopes are, provided they are good, the better they will be for this Effect; yet somewhat of Limitation is requisite, at least, that they be shorter than certain Limits for several Distances. These may be as follows: For 1 Mile, 1 Foot; For 2 Miles, 2 Foot; For 3 Miles, 3 ½ Foot; For 4 Miles, 4 ½ Foot; For 5 Miles, 5 Foot 10 Inch. ; For 6 , 7 ½ Foot; For 7 , 8 Foot 9 Inch. ; For 8, 10 ½ Foot; For 10 Miles , 13 Foot, and so forward. One of these Telescopes must be fix'd at each extreme Station, and two of them in each intermediate; so that a Man, for each Glass, sitting and looking through them, may plainly discover what is done in the next adjoining Station; and, with his Pen, write down on Paper the Character there exposed, in their due Order; so that there ought to be two Persons at each extreme Station, and three at each intermediate; so that, at the same Time, Intelligence may be convey'd forwards and backwards.

NEXT, there must be certain times agreed on, when the Correspondents are to expect; or else there must be set at the Top of the Pole], in the Morning, the Hour appointed by either of the Correspondents, for acting that Day; if the Hour be appointed, Pendulum Clocks may adjust the Moment of Expectation and Observing. And the same may serve for all other intermediate Correspondents.

NEXT, there must be a convenient Apparatus of Characters, whereby to communicate any Thing with great Ease, Distinctness, and Secrecy. There must be therefore, at least, as many distinct Characters as there are necessary Letters in the Alphabet that is made use of (as is expressed in Fig. 1) And those must be either Day Characters or Night Characters: if they are to be made use of in the Day-time, they may all be made of three slit Deals, moving in the Manner I here shew, and of Bigness convenient for the several Distances of the Stations for which they are made, that they may

be visible through the Telescope of the next Station. Any one of which Characters may signify any one Letter of the Alphabet, and the whole Alphabet may be varied 10,000 Ways; so that none but the two extreme Correspondents shall be able to discover the Information conveyed, which I shall not now insist on, because it does more properly belong to Cruptography[sic]. If the Characters are for the Night, then they may be made with Links, or other lights, disposed in a certain Order, which may be veiled, or discovered, according to the Method of the Character agreed on; by which, all sorts of Letters may be discovered clearly, and without Ambiguity.

THERE may be various Contrivances to facilitate and expedite the Way of displaying and exposing these Characters to view, and of withdrawing, or hiding them from the Sight; but this I here shew, I conceive, will be as easy and simple as any: All which may be exposed at the Top of a high Pole, and by two small Lines moved at the Bottom, so as to represent any Character.

By these Contrivances, the Characters may be shifted almost as fast, as the same may be written; so that a great Quantity of Intelligence may be, in a very short Time, communicated.

THERE will also requisite several other Characters, which may, for Expedition, express a whole Sentence, to be continually made use of, whilst the Correspondents are attentive and communicating. The Sentences to be expressed by one Character may be such as these, in Fig. 2.

I am ready to communicate. I am ready to observe . I shall be ready presently. I see plainly what you shew. Shew the last again. Not too fast. Shew faster. Answer me presently. Dixi. Make Haste to communicate this to the next Correspondent. I stay for an Answer; and the like.

ALL which may be expressed by several single Characters, to be exposed on the Top of the Poles, by themselves, in the following Manner, so as no Confusion may be created thereby.

I COULD instance in a hundred Ways of facilitating the Method of performing this Design with the more Dexterity and Quickness, and with little Charge; but that, I think, will be needless at present, since whensoever such a Way of Correspondence shall be put into Practice, those, and many more than I can think of at present, will of themselves occur; so that I do not in the least doubt, but that with a little Practice thereof, all Things may be made so convenient, that the same Character may be seen at Paris, within a Minute after it hath been exposed at London, and the like in Proportion for greater Distances; and that the Characters may be exposed so quick after one another, that a Composer shall not much exceed the Exposer in Swiftness. And so great Expedition may not only be performed at the Distance of one Station, but of a hundred; for supposing all Things ready, at all those several Stations, for Observing and Exposing, as fast as the second Observer doth read the Characters of the first Exposer ; the second Exposer will display them to the Observer of the 3d Station, whose Exposer will likewise display them for the 4th Observer, as fast as his Observer doth name them to him, or write them down.

THERE may be many Objections brought against this Way of Communication; and so many the more, because the Thing has not yet been put in Practice. But, I think, there can hardly be any so great, as may not easily be answered and obviated.

THERE may be many Uses of this Contrivance, wherein it will exceed any Thing of this Kind yet practiced, but I shall not now spend Time to enumerate them; only in two Cases, it may be of inestimable Use. The first is for Cities or Towns besieged, and the second is for Ships upon the Sea; in both which Cases, it may be practiced with great Certainty, Security, and Expedition.

LET ABC (Fig. 1) represent three very long Masts or Poles erected. E the Top-piece, that joins them all together. D, a Screen, behind which, all the Deal-board Characters hang upon certain Rods or Lines, and may (by the help of small Lines coming down from the Bottom of each of them) be exposed at F, or drawn back again behind D, as Occasion shall be. G is the Character for a Sentence agreed on, &c.

ALL the Alphabet, or requisite Characters, may be distinctly, and without Ambiguity, expressed. Such a Disposition as this, which I have here describ'd, I think, will be sufficient.