clocks at the observatories; and Baron Zach states that the arc of longitude between Buda and Paris could have been determined by four stations, eight signals (? six), and three clocks. Before this, 1804, Baron Zach had exploded charges of six ounces of powder on the Brocken, which were seen on the Keulenberg, at the distance of 151 miles.

Common rockets were tried from Greenwich Observatory in 1775, but not being visible for more than six or eight miles, they are useless for such purposes. The Congreve rockets, however, more especially those improved by Captain Schumacher, a Danish officer, entrusted by the King of Denmark with his rocket department, were found to answer much better, as it was announced in the Danish papers, in 1819, that rockets fired as signals on the small island of Heelm, on the Categat, had been observed with a telescope by Captain Schumacher's brother, the celebrated astronomer at Copenhagen, a distance of twenty leagues.

Baron Zach states (1822) that Baron D'Augustin, a colonel of the Austrian Artillery, had invented rockets which were capable of ascending to the height of nearly 11,000 feet, or more than two miles; but that it was difficult to follow them in their ascent to the point of explosion with powerful telescopes of a small field of view.

It does not appear that Sir W. Congreve's experiment led to any immediate application of fire signals to the purposes of the survey, although the subject was frequently discussed by General Colby at a later period, and he always anticipated the use of Drum

mond's Light in determining the differences of longitude of important stations, distant from each other, such as the observatories of Greenwich and Dublin. The French have subsequently used rockets, though with only partial success, in the great arc perpendicular to the meridian, between Brest and Strasbourg, before referred to, and the English and French, in determining the difference of longitude between Greenwich and Paris, as described by Sir John Herschel, in "The Philosophical Transactions for 1826." In 1839, the Rev. Dr. Romney Robinson, the well-known astronomer of the Armagh Observatory, determined in this manner the difference of longitude of the observatories of Armagh and Dublin, the rockets having been fired from the top of Slieve Gullion, a high mountain near Newry, and eighteen miles from Armagh. This mountain is 1,893 feet above the sea level, and though visible at Dublin, fifty-seven miles from it, is cut off from Armagh by an intervening ridge, so that the rockets were required to ascend more than 800 feet in order to be visible on explosion. All the details of elevation, bearing of hills, angles of elevation or depression, effects of refraction necessary to facilitate the operation were furnished by Lieutenant Larcom (now Major Larcom, Under Secretary for Ireland), from the documents of the Ordnance Survey; and it is not a little gratifying to those who have been connected with that great work to read the disinterested testimony of so able a judge to its general excellence. "It is," he says, "really wonderful how completely every

undulation of the ground has been registered in the survey. The altitudes sent to me, which must have been computed from the general sections, agree with observations in the most extraordinary way. A fact of another kind will show such persons as may not be acquainted with these things the precision of the Ordnance Survey. I set a telescope to the azimuth given for Slieve Gullion, and ascended the intervening hill with a theodolite, which I moved till, by signal from the observatory, it was in the line; then I took, with the theodolite, the angle between the telescope and the pile on the mountain top, where our rockets were to be fired; it proved 180 deg. exactly, or the three points were in one right line.”

It should be remembered that the survey thus commended had been planned and conducted by General Colby; and that the precision recorded had been attained by surveying and plotting on a scale of six inches to a mile-a consideration which ought not to be lost sight of in discussing the proper scales for other surveys, or in considering the comparative expense of this and other surveys. That a survey furnished with accurate levels, so numerous as to exhibit every undulation or swell of the surface, could be considered by any one useless for the preliminary examination of civil engineers, seems indeed scarcely reconcilable with the experience of Ireland, or with the ordinary deductions of reason. The Ordnance will doubtless be also gratified by the testimony which Dr. Robinson bears to the excellence of the rockets furnished from the Woolwich Arsenal

"The rockets," he observes, "were remarkably good; not one burst, which certainly is a singular contrast to the French rockets in Sir J. Herschel's and Colonel Bonne's operations. Their average rise, on the only evening I measured it, was 800 yards; they had, however, only four ounces of powder, but the part of the case which contained it weighed six ounces more, so that they actually carried a greater weight than the French." Much yet remains to be done to perfect the rocket as a military engine; but, even as the means of conveying signals at times and under circumstances when all other modes of telegraphing would be useless, their value should be deemed inestimable, when it is considered that a pound rocket will rise 1400 feet, a two-pound rocket will rise to that height and carry with it four ounces of bursting powder, and that Dr. Robinson succeeded in making some not heavier than the above, which carried four ounces of powder and rose to the height of 4500 feet.

The present opinion of British geodesists on the value of the signal system of determining differences of longitude is thus given by Captain Yolland, R.E., in his able and elaborate treatise on geodesy, which forms a second part of the third volume of the new mathematical course for the R. M. Academy: "The method by instantaneous signals consists in two observers, at A and B, accurately noting the correct time by the chronometers of the appearance or disappearance of the signal agreed on to be exhibited from some intermediate station, C, visible from both

A and B. A careful person being stationed at the intermediate station C, with instructions to fire the gunpowder or signal rocket, or to cover or uncover the Drummond, Bengal, or blue light, at stated times, and to repeat the signal agreed on at certain intervals of time when the weather is favourable, so that the observers at A and B may be on the look-out at the proper time for the explosions, appearance or disappearance of the signal lights. A set of such observations will furnish a series of clock times for A and B, which may all be severally reduced to sidereal times from the observed times of transit of the selected stars, and the difference between such sidereal times is the resulting difference of longitude furnished by each particular observation. It is probable that the precision of the result, obtained by this method of determining the difference of longitude is greater than in that of any other process which has yet been followed." To these remarks of Captain Yolland, a caution should, however, be added, that the determination of the absolute times at the two terminal stations should be effected in the most perfect manner, and, if possible, by fixed instruments; but as regards intermediate stations, intended merely to transmit the signals by observing the relative difference of time of their exhibition, a good watch showing seconds, and which keeps a tolerably steady rate, is sufficient, as the interval of time between the exhibition of the signals of the two adjacent stations intended to be observed by one person in succession, ought not to exceed a few minutes.