were small apertures emitting flame, and rapidly appearing and disappearing in succession. The light was very intense and splendid. On the north of the great fissure of the crater above alluded to, the recent lava assumed the aspect of basaltic columns. On the 27th of December, a cavern near Mauro was covered with a white incrustation of salt, sublimed from below; its quantity was so considerable, that 50 or 60 people made a profitable occupation of collecting it; for this purpose they either broke the stones, or scraped off the saline matter, and replaced them in their former situations, and a day or two afterward they became again covered as before. We often saw the deposition of this sublimate, which I am induced to believe required the presence of air for its formation, for it only existed near the surface, or in cavities open to the access of atmospheric air. The same observation applies to the beautiful specimens of sublimed oxide of iron (fer oligiste.) Various other sublimates were deposited upon the lava, but in much smaller quantity; their colours were chiefly yellow, red, and green; they were most abundant near the large crater; the yellow and red were deliquescent; but the yellow and green permanent. The smell of muriatic acid, though frequently perceived near the large burning orifice of the mountain, was never observed in the lava of Mauro. The sand ejected during this eruption was of two kinds; one red and in large grains, found upon the west ridge of the mountain; the other of a colour approaching to violet, and much finer. On the 25th the air was dark; there was not a breath of wind; but the sea on the coast was extremely agitated. In the evening there was a hail storm accompanied by red sand. ART. II. Instructions for the Adjustments and Use of the Instruments intended for the Northern Expeditions.* Captain Kater's directions for the use of the Instruments executed under his superintendance. ONE of the many objects of scientific research which present themselves on the present occasion, is the length of the pendulum vibrating seconds in a high northern latitude; and from the excellence of the instruments provided, we may confidently hope for results on this and on various other subjects, far more satisfactory than any that have yet been obtained. A Clock is sent out with each Expedition, the pendulum of which, cast in one solid mass, vibrates on a blunt knife edge resting in longitudinal sections, of hollow cylinders of agate. The points to be determined are, the number of vibrations made by the pendulum of this clock in a certain known interval of time; the arc in which the vibrations are performed; the temperature; the height of the barometer; the latitude and longitude of the place of observation; and (if practicable) its elevation above the level of the sea. A Transit accompanies each clock, the adjustments of which are, To place the vertical wire perpendicular to the horizon; and The line of collimation at right angles to the axis. The level requires no correction, it having been permanently adjusted by the maker. Slide the eye-piece in or out till the wires are seen distinctly. Direct the telescope to some distant and well-defined small object, and turn the milled head which is on the side of the transit till the object is seen with perfect distinctness. Place the level on the axis, and bring the bubble to the middle by * These Instructions were printed by desire of the Council of the Royal Society, and copies were distributed among the proper persons employed on the above occasion. the screw which elevates or depresses one of the Ys. The axis of the transit will then be parallel to the horizon. Having brought the object to the central vertical wire by means of the screws which act horizontally on one of the Ys, observe whether the same part of the object is covered by the wire whilst the telescope is elevated or depressed, and if not, correct half the apparent deviation by turning round the cell which contains the wires. The vertical wire covering some well-defined distant object, take the instrument out of the Ys, and carefully invert it, when, if the wire no longer covers the same part of the object, correct half the error by means of the screws which act horizontally upon the wires, unscrewing the one and screwing the other; and the remaining half by the screws which act horizontally on one of the Ys. Repeat this operation till the vertical wire covers the same part of the object in both positions of the telescope; the line of collimation will then be perpendicular to the axis. These adjustments once made will seldom vary. Of the Observations necessary to obtain the Number of Vibrations made by the Pendulum of the Clock during a certain interval. Screw the triangular support of the clock very firmly together, and having taken off the head, fix the clock-case to the support by the screws for that purpose. Bring the bubble of the level which is near the agates, to the middle by means of the screw which acts on the piece projecting behind from the triangular support, taking particular care that the three legs of the support rest on a very firm foundation, as on the stability of this will depend in a great measure the accuracy of the results. Next, see that the fork which is connected by a joint to the crutch is perpendicular, as it would otherwise be liable to injury. The pendulum is now to be taken from its case, and carefully passed up through the aperture which is made to admit it at the back of the clock, and gently lodged on the brass near the agate. In this part of the operation the most minute caution is necessary to keep the pendulum as nearly perpendicular as possible, in order that the weight of the ball may not bend the rod. The pendulum may now be held by the rod with one hand and raised, whilst the other hand is employed in guiding the knife edge, so that it may be properly lodged in the agate cylinders. The figure engraved on the ball of the pendulum should be in front. Turn up the fork attached to the crutch, so that it may embrace the rod of the pendulum. Hang on the weight, wind up the clock, replace the head of the case, and then gently set the pendulum in motion. Listen to the beats of the pendulum, and if they are not made at equal intervals, unscrew the lower screw which attaches the clock case to the triangular support, and move the clock case a very little to the right or left, till the beats are perfectly equalized. When this is effected, let the screws which attach the clock case to the support be firmly tightened. The thermometer is now to be hung in its place in the clock case; and the door being open, the whole is to be left for some hours, that the pendulum may acquire the temperature of the surrounding atmosphere. During this interval, the transit instrument may be put up, and screwed to the firm block provided for its support. When the pendulum may be supposed to have attained a steady temperature, the next step is to compare the clock with a chronometer which I shall call No. 1, distinguishing the others by Numbers 2, 3, &c. Shut the door of the clock case, and set down the hour shown by the clock, and the minute which is about to be completed. Place the ear close to the clock, and begin counting the seconds, looking at the same time at the chronometer No. 1, and on counting 60 seconds, which will complete the minute of the clock registered, mark carefully the second, and fraction of a second shown by the chronometer, and set this down together with the minute and hour. Open the door of the clock case, and observe the height of the thermometer, taking it by estimation to the tenth of a degree. Next, remark the extent of the arc of vibration of the pendulum, for which purpose a black line is drawn at the bottom of the ball. The mean of the arcs observed to the right and left, will be the true arc of vibration to be registered, and this must be estimated to the tenth of each division of the arc. This completes the comparison of the clock with the chronometer No. 1, and the clock case is now to be shut. Compare the chronometer No. 1, with No. 2, No. 3, &c., and according as these last are faster or slower, add or subtract their respective differences to the time shown by chronometer No. 1, before registered. The results will be the times which were shown by the chronometer No. 2, &c. when the clock was compared with No. 1, which times are to be registered in appropriate columns, the rate of each chronometer being noted at the head of its column. The height of the barometer may also be now registered. The temperature and arc of vibration should be observed and set down at equal intervals of two or three hours, in order to obtain a true mean of both. At the end of ten or twelve hours, the comparison of the chronometer No. 1. with the clock and with the other chronometers is to be repeated, and the observations will be continued in like manner during as long a period as circumstances may permit. The following form may be found convenient for registering the observations. At Lat. Date. Time by the Clock. Time by Chron. Time by Chron. Time by Chron. No. 2. gaining No. 3. gaining No. 1. gaining (or losing.) (or losing.) Tempera ture. |