These numbers, while vividly demonstrating the amount of disturbance of the yacht's deck, also indicate the very compound nature of the rolling of a vessel at sea; for, through the individual undulations, there run so evidently others of a longer period, and through those, others still longer. Indeed, I was never able to get any regularly recurring series of numbers; and after the passage of any given maxima and minima, there were sure to appear in the course of time others of yet higher order and greater intensity. On one occasion, after long watching, I caught a roll of 40°; and this did while rolls of 31° did not, set things generally rolling and tumbling about in the cabin. As this species of actual and social disturbance below deck only occurred 3 or 4 times in the day, we may assume that there are amongst the waves of the sea undulations having a period of several hours, and with their crests from 30 to 40 miles apart.

Such a result shows that no practical increase in the size of ships will altogether do away with their rolling at sea—that rolling which tends so to the discomfiture of the passengers, and the impossibility of taking telescopic observations. Neither can the old-fashioned plan of swinging tables avail

anything. This was shown from principle in the paper of last spring, and was found in practice on this voyage; for, as may be seen in the tabular representation of the observations, measures were made of the rolling of the dinner tables on board the yacht; the tables being hung on pivots with a heavy weight below. They answered their purpose famously as dinner tables for rough sea weather, for the plates and dishes kept their places marvellously. But when tested by the clinometer, or by the sun, they were found to be continually oscillating, through a less angle certainly than the yacht was rolling to, but so much more quickly, that the angular amount, multiplied by the number of vibrations, generally gave a larger total quantity of displacement in a minute for the swinging table than for the yacht. In the little yacht the motions were doubtless much more excessive than in the

larger class of ships now employed in long voyages; but if so, the occasion was all the more proper for testing the "Free Revolver Stand," as a means of eliminating from an astronomical instrument, both the rolls and the pitches of a vessel.

One of these instruments, on the principle described to the Society last spring, was on board, and was set in action on the 3d of July.

The telescope mounted was a small one, magnifying 7 times; and having an eye-hole of one-eighth of an inch. The revolver was 1 foot in diameter, weighed 10 lbs., and was driven by two sets of wheel-work acting on either side of the spindle. The apparatus was arranged inside a sort of observatory box, into which the observer could insert his head and two hands, while openings for observation could also be made. The only convenient place on the deck for erecting the instrument was near the stern, where was the maximum of the up-and-down movement, and the look-out was over the quarter, with the full effect of the rolling; all this, however, was well for testing the new principle.

The sailors were called up to work the driving-wheels, and when they had got up about 80 revolutions per second in the revolver, the wheels were withdrawn, the gymbal rings unclamped, and I had the satisfaction for the first time that any one had had, of looking at sea through a telescope steadied by

the same mechanical effort as that which preserves the constancy of the earth's axis in space.

The following practical points were soon ascertained: first, that in spite of the egregious rolling of the yacht, which must be partaken of by the observer, but is happily not partaken of by a telescope mounted as ours, there is no difficulty in keeping the eye en rapport with the eye-piece, although in the present case the size of the eye-hole is extremely small; second, that the position of the platform of the telescope can be adjusted with great nicety in any direction as respects deviation from the horizontal plane, by simply pressing on one of the gymbal rings, in a direction 90° removed from that in which it is desired that motion should take place; and, third, that the telescope can be adjusted in azimuth or altitude by rack and pinion, without sensible derangement to the position of the platform.

Having performed these adjustments, and brought the horizon of the sea into the field of view, I was delighted to find it remain there, absolutely uninfluenced by the rolling and pitching of the yacht; nay, it even remained bisected on the wire sufficiently long for the captain, the first and second mates, and several of the sailors, to look in and bear witness of the fact. They saw this consummation long desired at sea, and they took kindly to the instrument, though it was an innovation on nautical practice.

In order to perfect the balance of some parts of the machine, which had left its maker's shop rather unfinished, I kept the sailors for several days working the driving-wheels while I was experimenting; they laboured at them with enthusiasm, indeed, rather too much so, for one unfortunate evening, when the instrument was so nearly perfect, that I hoped in the course of the night to essay the grand nautical problem of observing an eclipse of one of Jupiter's satellites, the sailors, impatient of the difficulty of getting up immediately the full speed, clapped on a rope to the handle of the driving-wheel, and pulled away in such style, that they twisted the strong steel driving-axle in two, and were themselves laid flat along the deck.

The practical sufficiency, however, of the single-wheel re

volver stand, for correcting the rolling and pitching of a ship at sea, had been abundantly proved. And more than this, the azimuthal deviations of the yacht, which had been previously neglected as comparatively trifling, so long as rolling and pitching existed, were now found, when these were eliminated, and yawings left as the sole residual, to be absolutely so large, and so annoying to telescopic vision, as imperatively to demand attention and correction by the same revolver method also. On looking into the telescope, for instance, on the above-described stand, no change of plane from the horizon of the sea could be discovered; but any point in that plane, as a distant ship, was oscillating backwards and forwards in the horizontal direction twenty times in a minute; this being the effect of the azimuthal changes of the position of the yacht not having been eliminated as had been the rolling and the pitching.

The application of a second revolver to the apparatus which was employed on board, to enable it to eliminate all the angular disturbances of the yacht would be easy, as indeed I described in my paper of last spring, and showed in the case of a working model, and such an addition should be a necessary preliminary to any further trials at sea.

A Short Description of the Ganges Canal. By THOMAS LOGIN, Esq., C.E., F.R.S.E., Executive Engineer in Pegu. With Illustrative Diagrams.*

India has long been considered one of the most fertile countries in the world, and deservedly so; yet here also famines visit its thickly-inhabited plains.

In the years 1837-38, comparatively little rain fell in Northern India, and so great was the famine, that Government had to remit one million sterling of revenue, and thousands of the inhabitants perished from starvation. Previous to this, Colonel Colvin, Superintendent-General of Canals, requested Captain (now Colonel) Sir P. T. Cautley to examine the country below where the Solani falls into the Ganges, to ascertain if it was practicable to draw off a portion of the water of the Ganges for irrigation. The report, however, was unfavourable, and the subject lay in abeyance till the famine attracted the serious attention of the Government to the subject.

Sir P. T. Cautley now submitted a project, in which not only a portion, but all the waters of the Ganges which escape from the Himalayas in the dry season might be made to flow down the Doab, and thus render the recurrence of a famine an impossibility.

Lord Auckland, then Governor-General, manifested a deep interest in the project; but owing to changes in the Government, and expensive wars, it was not till 1847, when Lord Hardinge visited Roorkee, that orders were given to appoint an adequate staff of officers, namely, one director, six executive engineers, fifteen deputy superintendent engineers, and about thirty European overseers; and as at least one year was required to survey and mark out the line, the greater portion of this large work has been completed in seven years. My reason for mentioning this is, because it has been said that the rates on the Ganges Canal were low, as time was of no value.

Having thus given a brief history of the work, it is time to consider its uses, &c.

* Read before the Society, 13th April 1857.