One of the most ingenious and ardent improvers of the telegraph, is our countryman Colonel Pasley, who published in 1810 an account of the polygram matie telegraph for day signals, which he had in vented in 1803. He proposed to erect four posts, with arms complete, at every signal station, as shown in Fig. 11. In 1810, he described another on a new construction, shown in Fig. 12. This is precisely similar to the first, excepting that the pairs of arms used are placed upon one lofty post, instead of several short ones.
Considering the shutter principles inferior to that of the Semaphoric arms, Colonel Macdonald proposed in 1808, the machine shown in Fig. 13, the construction of which, and the reason for adopting it, are thus given by himself. "As three shutters give only seven mutations, it was found necessary to have recourse to four, which furnish fifteen changes. The four shutters were placed in a frame over each other, and worked in the usual manner. Shutters 1, 2, 3, and 4 shut in succession, for the highest gave numerals I, 2, 3, and 4. To have the remaining numerals, it was requisite to combine or to exhibit two shutters closed together, or at the same time. Thus shutters 1 and 4 gave numeral 5. Shutters 2 and 4 closed, expressed numeral 6. Shutters 3 and 4 closed, yielded figure 7. The two upper shutters closed, represented numeral 8. Shut ters 1 and 3 closed, denoted figure 9, and the two middle shutters closed, represented the 0 or cypher. As four shutters, acting as above, in a frame, would furnish a telegraph capable of giving only one figure at a lime, it was requisite to have three conjoined frames, with four shutters in each frame, in order to be enabled to have the places of units, lens, and hundreds; or in other words, to be able to express any three figures simultaneously. On this simple principle I constructed my twelve shutter telegraph for fixed stations. Four of the five of the remaining combinations in each of the three sets of co-operat ing sets or frames, are given by exhibiting three shutters closed, as shutters 1, 2, and 3; 1, 2 and 4; 1, 3 and 4; 2, 3 and 4; the last or fifteenth mutation being furnished by closing all the four shutters. * * * There is a circular board over the middles or of shutters, and the application of it doubles power in particular instances, and is other wise essentially beneficial."
In the year 1816, Sir Home Popham, who had distinghished himself by his improvements on navy signals, introduced a new semaphore, which was adopted by the Admiralty and substituted in place of the shutter telegraph. It is shown in Fig. 14, and is nothing more than two arms moving round separate centres upon the same post.
In 1818 Lieut. Colonel Macdonald invented and described a ball and six shutter telegraphs calcu lated to express any three figures simultaneously. It has an auxiliary ball above the middle row of shutters, and is surmounted by a semaphore for ex pressing the classes of words. The same ingenious author invented in 1817 a six ball and three figure telegraph, which gives 4095 combinations, and when conjoined with two of the semaphoric powers, not less than 1,048,575 combinations. See Fig. 16.
The most recent, and probably the best of all the telegraphs that have yet been proposed, is the Uni versal Telegraph, invented in 1822 by Colonel Pasley, who has given the following description of it: For the day signals the telegraph consists of an upright post of moderate height, of two movable arms fixed on the same pivot, near the top of it, and of a mark called the indicator on one side of it. See Fig. 17.
Each arm can exhibit the seven positions 1, 2, 3, 4, 5, 6, and 7, exclusive of its quiescent position, called "the stop," in which it points vertically downwards, and is obscured by the post. Fig. 17 represents the telegraph exhibiting the sign 17, the other positions, of which the arms are capable be ing dotted. The indicator merely serves to distin guish the low numbers 1, 2, and 3, from the high numbers 7, 6, and 5, so that this telegraph is not, like most others that have been proposed, liable to ambiguity or error, when viewed from different points in contrary directions*.
The use of the indicator will appear more evident on considering the resemblance between the small Roman letters b and d, or p and q, which if viewed in contrary directions, like telegraphic signs, could never be distinguished, one from the other, without some additional mark.