In respect to the dimensions proper for the parts of the Universal Telegraph, we ascertained by ex periment, that the arms for the clay signals should be about one foot in length per mile, in order to be distinguished by a common portable telescope of moderate power. This length is computed from the centre of motion to the end of the arm, not in cluding the small part beyond the centre, called the head. By the above rule, a telegraphic arm, of six feet in length, may suffice for stations six miles apart ; but generally speaking, in telegraphs in tended for permanent stations, where the saving of the weight is less an object, it may be considered best to add a little to the dimension thus found.
The width of the arm need not exceed of its length, and should not be less than lth or 8th of the same dimension. The indicator for the day sig nals should be of the same width, but only lths of the arm in length.
The height of the post should be such, that men, or other movable objects, passing near it, should not obscure the indicator, or arms, when the tele graph is erected on the deck of a ship, or in the field. But when placed on the roof a permanent signal-house, the projecting part of the post need not exceed the telegraphic arm, by more than !rds of the length of the latter.
It is desirable in all cases, that the telegraphic post should be capable of turning, so as to exhibit the arms in various directions. On board ship it must also be occasionally lowered. Hence it be comes necessary to step it upon a simple open cir cular joint of iron, fixed to the ship's side near the deck, and to secure it by an iron clamp, also of a circular form, attached to the rail, nearly in the same manner as the ensign staff of a man-of-war is usually fitted.
The telegraphs hitherto constructed upon this principle, are of two sizes. One having arms of 51 feet in length, with the lantern pivots placed 61 feet from the centre of motion. The other having arms of 21 feet in length only, with the lantern piv ots three feet two inches from the centre of motion. The former are of a size suited to the largest class of men-of-war. The latter are perfectly portable, as the whole apparatus, including the night indica tor, lanterns, &c. does not weigh more than 34 lbs. In clear weather, these small telegraphs make sig nals distinctly at the distance of three miles.
Supposing that telegraphic signals should be re quired on a sudden emergency, in some situation where there may not be time and means for making well finished telegraphs, in the manner that has been described, I have ascertained by experiment, that the most expeditious and satisfactory arrange ment will always be to copy the regular construc tion, as closely as circumstances will permit. A
post, with two planks for the arms, each worked merely by a couple of strings without pulleys, will constitute a day telegraph, and the addition of lan terns &c. will convert the same simple apparatus into a nocturnal telegraph. In both cases, the arms must be counterpoised by wood or iron, and also by weights, but in a ruder manner than was before de scribed. To adopt balls or flags for day signals, or an immovable rectangular frame, with ropes and pulleys, for supporting the lanterns for night signals, which are the only other expedients that suggest themselves as a temporary arrangement, will, on trial, be found much less satisfactory than the rudest attempt at the counterpoised telegraphic arm.
It is well known, that telegraphs should generally be painted black, and that for permanent stations, they should always be erected, if possible, upon heights having no back ground.
Several telegraphic dictionaries have been com posed by different authors, but of these, the one now used in the Royal Navy, which was compiled by the late Rear Admiral Sir Home Popham, appears, upon the whole, to be the most judicious. The number of words and sentences contained in it does not exceed 13,000 ; and yet there is seldom a defi ciency of any useful word. Another author has composed a dictionary of a similar nature, contain ing upward of 31,000 words and phrases : and a third has composed a work containing more than 149,000 words, phrases and sentences. It may be observed, in regard to this subject, that the exten sion of a telegraphic dictionary beyond a certain limit is an evil, because in proportion to the number and length of the sentences contained in it, it be comes so much the more difficult to find any of them, without a vast loss of time.
Hence the advantages held out by the author of any very voluminous telegraphic dictionary, must always be in a great measure nugatory, unless the place of every phrase or sentence contained in it, could be known by intuition, which is impossible.