By JAMES GLAISHER, Esq., F.R.S.

THE variations in the readings of the barometer at the Royal Observatory, Greenwich which are delineated in the annexed diagram include a period of forty days, or from 27th October to 5th December, 1863; and during this interval of time there occurred at least three severe and extraordinary gales of wind. The approach of these storms, and the generally disturbed state of the atmosphere, are well shown by the great and sudden depressions of the mercurial column, fully showing the great value of barometric indications, and the advantage of an unbounded reliance on its prognostications, trust the following remarks will prove to fishermen, and others who may be placed in positions to consult the truly valuable indications of these instruments, one of which is placed at many of the stations of the ROYAL NATIONAL LIFE-BOAT INSTITUTION, and at other places, the folly of neglecting the cautions given by the barometer; and also the absolute necessity which exists for combining instrumental indications with their own local weather estimates, drawn from natural and familiar sources.

The first of these storms occurred on the 30th October, and was in part indicated by the steady decrease of the barometer from 29-84 in. on the 27th to 29-10 in. on the 29th ; and although it increased slightly to 29 • 32 in. on the morning of the 30th, it was but to decrease with greater rapidity to 28-80 in. by 3 -30 P.M. This rapid decline of the readings of the barometer to the amount of half an inch in nine hours only, fully indicated the approaching squall; indeed, with the first symptoms of this rapid decrease, the wind commenced to blow strongly from the S.W. As early as 9'30 A.M. a gust of 15 Ibs. pressure on the square foot took place, which was accompanied by a heavy shower of rain ; but from 10 A.M. to 3 P.M. the pressures never exceeded 5 Ibs.; after 3 P.M. the wind became extremely violent, successively reaching pressures of 12, ] 3, and 17 Ibs.; and finally the extreme pressure of 29£ Ibs. was recorded at 3 "30 P.M., being coincident with the time of the minimum reading of the barometer.

This pressure was unprecedented as regards the Greenwich records, the extreme pressure in all preceding years from 1841 having been 28 Ibs.

The indications of the several self-recording instruments at the Royal Observatory, for the greater part of the 30th October, have been placed on a diagram, which more fully shows the variations of the barometer and temperature, the direction of the wind and its pressure per square foot. A glance at this diagram will show the coincidence of the lowest reading of the barometer with the extreme pressure of the wind; the sudden change of 90° in its direction, or from S.S.W. to W.N.W., and rapid decline of temperature of nearly 10°. (See Plate in proceedings of the British Meteorological Society.) After the readings of the barometer turned to increase, the wind gradually subsided. The diagram will show that the chief violence of the gale occurred between the hours of 3 P.M. and 4-20 P.M. On the following day the pressures of the wind occasionally reached 8 Ibs. and 9 Ibs. on the square foot, but the pressures generally varied between 2 Ibs. and 5 Ibs.

On the morning of the 2nd November, another minimum value of the readings of the barometer was reached—viz., 28 • 74 in., which was accompanied by a heavy rainfall, to the depth of half an inch, and by a S. and S.W. wind, which blew briskly, although not heavily.

The second storm which we have to notice occurred on the 21st November, and was foretold by a rapid decline of the barometer from 29'91 in. on the night of the 20th to 29 • 70 in. by the morning of the 2lst, and then to 29'44 in. by 5 P.M., accompanied, as before, by an extreme gust of 17jlbs., the great pressures continued only between 4 and 5 P.M. The third, and by far the most remarkable, storm, occurred on the 2nd December, and was amply presaged by the rapid fall of the barometer from 29 • 46 in.

on the night of the 1st, to 28 • 84 in. on the morning of the 2nd; the wind, however, did not begin to blow violently till 2 • 30 P.M., when a sudden gust of 9 Ibs. was recorded; from 2-30 till 2-50 P.M. the pressures varied from 5 Ibs. to 9 Ibs.; it then increased greatly in force; at 2 • 50 P.M. there was a pressure of 16 Ibs., and at 2-55 P.M.

one of 22£lbs.; from 3 P.M. to 3-20 P.M.

the pressures varied between 10 Ibs. and 15 Ibss with one gust to 20 Ibs.; afterwards no pressures to greater amounts than 7 Ibs.

and 8 Ibs. were registered. At the time of the extreme pressures in this gale, the readings of the barometer had turned to increase, and was reading 28'95 in. (the minimum having been 28'84 in. three hours previously); it then increased to 29-38 in. by 9-30 P.M.; and after 11 P.M.

again decreased rapidly, reaching a minimum value of 28 • 80 in. at 7 • 20 A.M. on the 3rd, and at this reading the barometer continued stationary through a period of one hour.

During this decided barometric decline, the wind had recommenced blowing strongly from 5 A.M. till 8 A.M., pressing with forces varying between 2 Ibs. and 8 Ibs.; at 8'10 A.M., whilst the barometer was at its lowest reading, the maximum pressure of this gale occurred—viz., 21 Ibs. on the square foot. The wind at the same time veered from S.W. to W., and continued from this quarter throughout the greater part of the day, still blowing with much strength, the pressures varying between 6 Ibs. and 12 Ibs.

till 6 P.M., and between 1 Ib. and 6 Ibs. till 1 A.M. on the morning of the 4th. In the meantime the barometer readings increased rapidly; as before stated, the minimum value continued till 8'15 A.M., and was 28 • 80 in.; by noon it had increased to 29-09 in.; by 6 P.M. to 29-40 in.; by midnight to 29'84 in.; at 4 A.M. on the 4th day, 30 in. was reached; and by noon, 30-22 in. Thus, within twenty-eight hours the extraordinary increase of 1-42 in. took place in the pressure of the atmosphere; and from 5 P.M. to 11 P.M. on the 3rd the increase took place at the almost uniform rate of 0'08 in. per hour.

The subordinate gales of this period have also been accompanied b corresponding barometric depressions, though in a less degree. In the preceding remarks, I think the connection existing between the decline of barometer readings and the then approaching gales are very plainly indicated.

We may surely learn from these examples —firstly, that the chief severity of a gale may be expected at or near the time of minimum barometer reading; and, secondly, that after the minimum has passed, the worst of the gale has passed, and that the storm will moderate as the barometer readings increase.