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of the degree of moisture. For this reason I took only the first and last quarters of the year, when the weather is usually dry, and found the means of the different years (see table No. 4). From this it will be seen that the days of highest dew-point in the winter half of the year are the 4th and 5th, before the new moon, and the 10th and 11th, after. This was so near a coincidence with two of the maximum days of rain, (viz. the 3rd before, and the 12th, after,) that little doubt could be entertained of the one being caused by the other. That neither the other two days, (viz. the 9th, before, and the 5th, after,) were maxima might be accounted for by peculiar circumstances. At this stage of the inquiry I was led to attempt to account for the phenomena by the following considerations :

1st. By the united testimony of every observer, the quantity of moisture in the air and the rain-fall become less, as we recede from the great eastern ocean. Thus if we could obtain the mean dew-point for every degree of longitude between Delhi and Dacca, the result would exhibit almost as regular an increase as in a list of temperatures between London and Algiers. I was aware too of the great increase of dew-point here whenever the wind came from the east, and that a continuance of it was usually followed by rain. I could not, therefore, but believe that the force of attraction of the moon as well as of the sun excited an influence over the aërial currents either in modifying their direction or changing it entirely. Mr. DANIELL remarks the excess of dew-point when the wind blows from the Atlantic (he is speaking of the climate of England), and the force of attraction of the moon is stated by D'ALEMBERT to be such as would create a westerly current of eight feet in a second, (see ROBISON'S Mechan. Phil.) But to render this force more apparent, we must have recourse to another consideration.

2ndly. The principal cause by which the air is affected is by the heating power of the sun, which expands a column of it,th part for each degree of Fahrenheit. Upon the ocean the heat is counteracted by constant evaporation; consequently, when a column of air, resting upon a surface of dry land, is heated by the sun, it becomes expanded, and of less specific gravity than an adjoining column in contact with the sea. Hence, as the heavier fluid will press upon and displace the lighter, a current flows in from the sea to the land. This diurnal phenomenon may be observed on almost every tropical coast. We have an annual instance of it in the great heats previous to the summer solstice, and the winds that follow them from every quarter of the ocean, the S. W. the S. E. and E., bringing with them abundant rain. This current must be strongest at the time of maximum heat of the day, and if we suppose

the moon in such a position as to act in conjunction with it, then the two forces would produce a great conjunction tide in the air. As the moon recedes eastward from the sun, it comes upon the meridian about 48 minutes later every day; so if the change happened at noon exactly, three days after it would be upon the meridian at 2h. 25m. P. M. As the time of maximum heat of the day is by Dr. BREWSTter 2h. 40m, P. M.we ought on this day to have the great conjunction tide, according to theory. But in comparing the actual tides of the ocean with the deductions from theory, we find that the phenomena occur one day and a half later than they ought to do; thus the greatest spring tide does not happen exactly at the conjunction of the sun and moon, but a day and a half later. Let us make a similar allowance in the case we are considering; then as the moon must be somewhat more than three days old when it is upon the meridian at 2h. 40m. P. M. add one day and a half to its age, and the greatest tide will be produced when it is nearly five days old. I venture to suggest this as the cause of the maximum fall of rain on the fifth day after the new moon, and the minimum of the barometer on that day. Of course, as air is distended and rendered lighter by being mixed with aqueous vapour, the presence of a great quantity of moisture (which would be the case in a current setting in from the ocean) is sufficient to account for the diminution of pressure.

Cor. 1st. This supposition may account for our spring showers happening as they usually do between 2 and 6 P. M. and probably at the time when the superior or inferior tides of the moon are near the meridian.

Cor. 2nd. If the supposition be true, then the excess of rain about the fifth day after the new moon will be greatest, when the heat is a maximum, when the sun is nearest the zenith, and when the moon is nearest the zenith. This would happen at Calcutta in the months of May and June. In the first four months the heating power is great, but the sun has south declination for most part of the time, and the moon too. In July the sun is near the zenith, and so is the moon, but the heating power is counteracted by constant evaporation. In the last three months of the rains the sun and moon are further from the zenith, and the heating power somewhat less than in July. I took therefore the sum of the rain that fell on the 3rd, 4th, 5th, 6th, and 7th days after the new moon, in each of the four periods, and compared each with the quantity that fell during the whole lunar period. Premising then that five days are to the whole lunar period as

10 : 59

I found the sum of rain that had fallen in the 5 days above mentioned, to be to the sum of the whole lunar period,—

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But besides the superior or direct tide of the moon, the inferior or opposition tide of the moon would be in conjunction with the greatest heat about the 9th day before the new moon. I took therefore the 9th day with three days before and three days after it, and found the tions the sums bore to the whole period in the same manner as above.

Seven days being to the whole lunar period,
The proportion was-

In the 1st four months,

May and June,.

July,...

Aug., Sept., Oct.,...

propor

::

10

:

42

::

10 : 40

::

10

:

40

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10

: 55

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The irregularity in the case of July probably arises from a sufficient series of years not having been taken. If instead of the quantities of rain we take the number of rainy days in the same periods, they give a ratio of 10: 40

With a view of ascertaining whether similar results were to be observed in the climate of Great Britain, I next made a table of the temperature at Edinburgh, for eight years, (from 1824 to 1831, both inclusive,) from the Edinburgh Philosophical Journal; to this I added a table for three years near London, (from Sept. 1819 to Sept. 1822,) which is to be found in DANIELL's Meteorological Essays, and the results are as follows: (see table No. 5;) taking the days as before, (viz. the 5th day after the new moon, and two days before and two days after it,) the ratio to the whole lunar period was as follows:

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It was to be supposed that in a high northern latitude, in the three last months of the year, when the heating power of the sun is very small, owing to the great moisture, and also the sun and moon (when it is near the change) have southern declination, that the joint effect of the heat and attractive force would be barely perceptible. There is, however, another cause of mistake. Though the mean time of maximum heat for the whole year is 2h. 40m. P. M. yet that time varies with the different seasons; in summer it is considerably later, in winter it is considerably earlier. I have not the book to refer to, but taking the 6th day in the summer months for the centre of the maxima, instead of the fifth, after the new moon, and the 2nd instead of the 5th for the last quarter, the ratios are as follows:

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Let us next compare the day of the moon's opposition, (viz. the 9th before the new moon,) and three days before and three days after, as was done in the former case.

The ratio of the amount to the whole lunar period was

In the first four months,

Summer months,..

Oct., Nov., Dec.,

......

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: 28

2

But taking the 13th before, instead of the 9th, (for the last quarter,) we get a ratio of We may observe then that the amount which falls in these days near the full moon is greatest in winter, when the moon near the full has north declination. On the contrary, in summer, the amount which falls near the new moon, when the moon at that season, and that age, has north declination, is the greatest. We may recollect that in the theory of the tides the height of the tide is said to vary as cos x (where x is the angular distance between the moon and zenith of the place). The above observations seem to point to a law somewhat similar. But of this I have yet to offer some further probability. have not here compared the number of rainy days as well as the quantities of rain fallen, but they tend to the same conclusions, though less decisively. Nor have I said any thing respecting the two other maxima on the 3rd day before, and 12th after, the new moon, as I have no probable cause to allege for them.

I

Let us then dismiss from our minds the idea of a sphere covered with a homogeneous fluid, and substitute that of a surface partly of dry land, and partly of water, the first covered with a stratum of air nearly dry, the last with a stratum saturated with moisture; and to carry on the comparison with the tides of the ocean, let us remember that we cannot measure the actual height of the tide, as in that case, but that if an observer, situated on the border of an estuary, were to endeavour to estimate the relative intensity of the currents flowing in from the open sea, by the quantity of salt contained in the water before him, then his case would be somewhat similar to ours, when we attempt to draw a like reference respecting the aërial currents from the heights of the dew-point. If he were to endeavour to conjecture the force of the floods from the country above, by measuring the quantity of earthy matter precipitated from the water, then he might expect to approximate to the truth about as much as we do when we attempt to infer the force of the current of air flowing in from the regions of the ocean, by the quantity of water precipitated. In both cases an approximation only can be expected.

Having gone thus far, the next step to be desired was to make a comparison between the heights of the dew-points at different ages of

the moon, and the heights of the tides of the ocean on the same days. The only table I could refer to was that given us by Mr. NOTON, (Jour. As. Soc. May, 1833,) of the tides in Bombay harbour, which answered tolerably well, as Bombay, as well as Calcutta, has considerable north latitude. The heights of the tides, day and night, both at change and full, are given there, as well as for three days after, and three days before, the day of change and full. I took, therefore, the average height of the tides in the seven days about each new and full moon, and compared them together. The first comparison was the day (or superior) tide of the new moon, and the day or inferior tide of the full It was as follows:

moon.

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It will be observed, that in the winter season, when the new moon has great southern declination, and the full moon has great northern declination, (or, in other words,) comes near the zenith of the place in question (Bombay), then the new-moon tide is not so high as the fullmoon tide; but, in the summer season, when the declinations are reversed, then are the ratios of the tides reversed also. I have marked with an asterisk the places where the ratios change. But we must here notice a remarkable anomaly in the lunar theory. The ratios we have observed above ought only to hold with direct or superior tides of both new and full moon, the reverse ought to hold with respect to the inferior tides of both. For instance, if the declination of the moon were 20° south, and consequently the vertex of her superior tide in 20° south latitude, the vertex of the inferior or opposite tide ought to be in 20° north latitude. So that in places to the north of the equator, as Bombay and Calcutta, the inferior tide would be very large when the superior tide was very small. Thus at Bombay, in winter, the night, or inferior tide, of new moon, ought to be very large; the day, or inferior tide, of full moon, very small: but on comparing them together, we find the reverse.

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