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March borrows frae April

Three days, and they are ill :
The first o' them is wun' an' weet;
The second it is snaw and sleet;
The third o' them is a peel-a-bane,
And freezes the wee bird's neb tae stane."

This is a capital description, giving by a few simple but strong touches the principal features of one of the bitter “norlan' blasts” or storms of the spring equinox; and it is all the more accurate inasmuch as these interruptions of temperature are more or less connected with storms. A period of cold weather also occurs in the middle of May. This cold is very extensively distributed, as is shown not only by its appearance in meteorological records, but also from its prominent position among the “weather saws” of every country in Europe. *

295. I have examined the temperature of Scotland for a number of years, and have shown t that the following interruptions occur from year to year, with very rare exceptions, in the annual march of the Scottish temperature :

(1. 7th to 10th February.

2. 11th to 14th April.

3. 9th to 14th May.
Six cold periods, .

4. 29th June to 4th July.
5. 6th to 11th August.
(6. 6th to 12th November.

(1. 12th to 15th July.
Three warm periods, 2. 12th to 15th August.

( 3. 3d to 9th December.

296. The interruptions which take place in July, August, November, and December have been particularly examined ; but since the same conclusions have been arrived at for them all, it will be enough to state the results of the examination of the November and December interruptions. The cold

* For an account of the cold week of May, see a paper of great elegance and learning read by Dr Arthur Mitchell at a meeting of the Scottish Meteorological Society, and since published in Good Words,' May 1866.

+ Journal of Scottish Meteorological Society, Nos. xiii., xiv., and xvi.

weather in the beginning of November is accompanied by an unusual prevalence of N. and N.W. winds, and by calms; by a diminution of the amount of the invisible vapour of the atmosphere; an increase in the rainfall, both as regards amount and frequency; and a more frequent occurrence of fogs. On the other hand, the mild weather of December is accompanied by an extraordinary predominance of southerly winds, and an almost total absence of northerly winds; an increase in the amount of the aqueous vapour, and a considerable increase in the rainfall, both in amount and frequency.

297. Hence, then, as regards the causes of the interruptions in the regular annual march of the temperature, we are led to one conclusion-viz., that they are determined and regulated by the wind. Nothing could present this in a clearer light than the winds which are found to occur during the November and December periods. Thus, during the cold period of November, winds from the N.E., N., and N.W. were in the proportion of 35 per cent, while winds from the S.E., S., and S.W. were 29 per cent. On the other hand, during the mild period of December, winds from the S.E., S., and S.W. were in the proportion of 69 per cent, while winds from the N.E., N., and N.W. were only 5 per cent.

298. But since the wind is only an effect or consequence resulting from differences between the atmospheric pressure in Scotland and that of neighbouring regions, we must look to the atmospheric pressure for an explanation of the interruptions of temperature. The pressure was examined with this view, and it was found to hold universally during the cold periods that pressure was higher to the north of Scotland and lower to the south, thus drawing over Scotland the polar current, and thereby depressing the temperature ; and during the warm periods, that pressure was higher in Scotland than in places to the north, thus drawing over the country the warm stream of the equatorial current. Thus the unusually cold or warm periods which occur with considerable regularity at certain times of the year have, so far as examined, been proved to depend on the relations of the polar and equatorial currents to each other. The circumstance that one of these great atmospheric currents, and not the other, prevails over this portion of the earth's surface at stated seasons is a valuable fact in meteorology, and the more so from the light it seems to cast on the periodicity of weatherchanges.

299. The commencement of each of these more anomalous periods is subject to variation from year to year; during the past fifty years some of them appeared every year between the dates specified, and none failed to make their appearance on more than five of the years. Being also of short duration, seldom exceeding six days, and most frequently limited to three or four days, they are sudden and striking—thus differing in character from a well-known anomaly which occurs in the latter end of January and earlier part of February.. In regard to this anomaly Principal Forbes, in his paper “On the Climate of Edinburgh,” remarks: “In most European instances it affects materially the mean temperature of February, which is very commonly far too high when contrasted with the general sweep of the annual curve. At Brussels, M. Quetelet finds an excess in the temperature of February above the general curve of at least 10.5. The same peculiarity may be noticed in the annual curves of London, Prague, St Petersburg, Vienna, and many other places, including the Great St Bernard.” It is probable that a careful examination of the monthly isobarometric curves will lead to a better knowledge of so important an anomaly, and of the causes of its occurrence when the region over which it spreads has been accurately defined.

300. From the principles enunciated it is plain that the climate of Scotland is ruled and determined by the relations which most commonly exist between its atmospheric pressure and the pressure of surrounding regions. Since the same principles are applicable to the whole atmosphere, it also follows that mean monthly isobarometric charts furnish us

144

TEMPERATURE-ITS RELATION TO ATMOSPHERIC PRESSURE.

with the key to the climates of all parts of the globe in the different seasons of the year; for the direction of prevailing winds thus becomes known, and these are warm or cold, and dry or wet, as determined by the regions from which they blow. This subject will be more fully stated in the chapter on Winds.

CHAPTER VIII.

THE MOISTURE OF THE ATMOSPHERE.

301. The Two Atmospheres of Air and Vapour.—The gaseous envelope surrounding the earth may be considered as composed of two distinct atmospheres—an atmosphere of dry air, and an atmosphere of vapour. The dry air (oxygen and nitrogen) is always a gas, and its quantity is constant from year to year; but the vapour of water does not always remain in the gaseous state, and the quantity present in the atmosphere is, by the processes of evaporation and condensation, varying every instant.

302. According to the strength of the force of cohesion drawing the particles of matter together, as compared with the repulsive energy of heat driving them asunder, so is the body solid, liquid, or gaseous. In solids and liquids the cohesive force is in excess, whilst in gases it is absent. It is the absence of cohesion which is the distinctive characteristic of gases and vapours. If a little water be poured into a vessel it will only rise to a certain level, and leave the rest of the vessel unoccupied. On the contrary, gases and vapours completely fill the vessel in which they are, showing that, instead of cohesion, there is a mutual repulsion among their particles. Since then, the particles constantly tend to recede from each other, it follows that they will exert an outward pressure on the sides of the vessel, and that the amount of this pressure will be proportioned to the repulsive force, or to the elasticity of the gas.

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