CHAPTER VI.

Indications of a Daily Lunar Tide.-Its trivial amount.-Supposed effect of the Changes of the Moon on the Weather.-First impression of the Recurring Monthly Periods in favour of the Moon. Mr. Howard's Investigation of the Lunar Influence, and his convictions in favour thereof.-Difference of result between Howard and the Greenwich Observations.—Mr. Howard's opinion respecting the Radiation of Heat from the Full Moon, and radiation from the Earth at New Moon.-Howard's Cycle of Eighteen Years in the Seasons of Britain.-The Recurring Periods are not coincident with the Lunar Phases.-Recurring Monthly Periods of the Thermometer not consonant with the Lunar Phases.The Metonic Cycle, or Lunar Period of Nineteen Years, investigated by comparing 1826 with 1845.-The days of New and Full Moon for the Year exhibit an analogy in the interval or periods of days to the interval of Recurring Periods.-Dr. Samuel Horsley's investigation of the WeatherInfluence of the Moon.-Schubler, Pilgrim, and Flaugergue's investigations of the same.-Volney's opinion.-La Marck's Weather Almanac, published a year beforehand, giving the Winds, Temperature, etc., for each Northern and Southern Aspect of the Moon.-Herschel's (so called) celebrated Weather Table, published 1803, calculated from the Moon's position, and the Weather to be produced thereby.--Mr. Orlando Whistlecraft's Weather Table, as deduced from the influence of the Moon at the Four Quarters.-Mr. Whistlecraft's days of Change of Weather for 1857, as deduced in part from the influence of the Moon at the Four Quarters.— The Critical Days (so called) of the Recurring Monthly Periods are not the days of Change of Weather, but the extreme states.-What is Change of Weather?

CHAPTER VI.

ON THE INFLUENCE OF THE MOON ON THE BAROMETER.

Greenwich Observatory Abstracts, 1845, page 33.

"The general fact of a daily lunar tide is indicated by the mean readings, increasing from 6h. W. to 6h. E., and diminishing from 2h. E. to 6h. W., hour angles respectively.

"It appears that the mean height of the barometer is increased by the moon's position in south declination.

"When the moon was at or near her mean distance, and particularly when coming nearer to the earth, the mean height of the barometer was greatest.

"It would seem that the mean pressure of the atmosphere was greatest when the moon was about fourteen days old."

SUCH are deductions from the most careful observations by those qualified to form an opinion, and from which I presume some effect of gravitation is produced upon the atmosphere by the moon, but the quantities are so extremely small as to escape ordinary observation. Moreover, they are so mingled up with the other predominating actions, as to require the most delicate

152

MINUTE QUANTITY OF THE ATMOSPHERIC TIDE.

analysis to detect. We are the more obliged to those who undertake such difficult problems, the extreme amount being only the six-hundredth part of an inch of the mercurial column of the barometer. Leaving such minutiæ for future inquiry, as they have no practical bearing, I proceed to the generally received and popular opinion of the influence of the moon upon the weather. What is it? has never been satisfactorily answered; but people look to the change of the moon effecting a change in the weather, with as much certainty as they look to a dial to know what o'clock it is. The weather changes and the moon changes, and they are satisfied, and all is right-the thing is certain, and proved to a demonstration. No one can deny the fact that the weather is very frequently changing, and the moon too, and if we take in a day or two before, and a day or two after each change, then it is a perfect and complete theory, which accounts for all the occurrences.

Such, however, are among the most vague of all popular errors. The moon may, or may not, have an influence on the weather; the question has never been proved on either side, and it remains as undecided as it ever was. When the tables of the recurring monthly periods were laid before an eminent and talented person, his first and almost immediate impression was, that the moon had the presiding power. The very circumstance of the monthly occurrence indicates, from the name itself, some connexion, but still there is very grea difficulty in establishing the fact, and the matter is by no means plain.

LUNAR INFLUENCE ON THE BAROMETER ONLY. 153

The Greenwich Reports, abounding with such minute and accurate observation and research, supply all the materials for investigation that can be required; but they only advert to the influence of the moon upon the barometer, implying almost as much as if it were the effect of gravitation. But the tables of the recurring monthly periods display facts upon a much larger scale, so large as to be obvious to all; indeed, they are the largest exhibited in the meteorological registers. They embrace the maximum and minimum, and, therefore, comprise the whole circle of facts. They present an order, or recurrence, of the same days or dates; but with the same, or with reverse, states and conditions of the atmosphere. There is no uniform effect, although a regular recurrence or succession of days.

"Influence of the Moon on the variable Pressure of the Atmosphere, on the Temperature, Winds, and Rain." -Such is the heading of a chapter in Mr. Howard's "Climate of London," so that we cannot attach the exclusive belief of the moon's influence to the people only. The Greenwich Observations sanction the idea of some lunar tide; but Mr. Howard, who was an intelligent and long continued and faithful observer, says (page 155, etc.):-" A clue is furnished to the chief difficulties, by the fact, now sufficiently ascertained, that the atmosphere is subject, like the liquid ocean, to the influence of the moon's gravity; and that from this cause, operating jointly with the sun's attractive power, it has its tides and currents, and I was induced to cast my reports on the weather into the form of lunar