will have been attained if we have conveyed to the reader a clear idea that observations at any one station give a section of the weather-changes which are shown in plan on successive synoptic charts; and that each self-recording instrument writes in its own language, and, as it were, in its own alphabet, the history of the weather for every day.

DESCRIPTIVE, OR NON-INSTRUMENTAL, RECORDS.

So far we have discussed the significance of instrumental records; but, however skilfully we may read those written traces, it is evident that there is still a great deal of weather about which they tell us nothing. No mechanical registration of pressure, temperature, or wind can ever make up for the want of a good verbal description of weather-sequence. No instrument can picture to us the various ways in which a blue sky can become overcast; whether the blue grows gradually pale and sickly, or whether great snaky-looking clouds seem irresistibly to embrace the whole heavens. Neither can it describe the delicate distinctions which our senses enable us to perceive in the way the wind blows. Our eyes tell us at a glance that a south-west wind raises a long sea, while a nor'-wester rakes the surface of the ocean into lines of foam; and that the fitful gusts of an impending shower drive little eddies along the dusty road.

In like manner, no short cloud-symbols, such as detached cloud, overcast, misty, or even the more detailed words-cirrus, cumulus, etc., can ever give more than a lifeless picture of the sky as we know it.

The old myth-makers excelled in their descriptions of weather. In their own peculiar figurative language we see reflected a vivid picture of cloud and thunderstorm which we can scarcely match in the more sober verbiage of modern times. The Greek poets knew the difference between the beneficent diurnal winds which sprang up at dawn and the dangerous blasts of an approaching thunderstorm; and never mistook the wind which sighed among the pinetops for the north-westerly squalls which tumbled the trees over the cliffs.

All that instrumental traces could tell of this would be deduced from seeing if the velocity-trace had some connection with the time of day, or if it was fitful, and that the direction-trace was also unsteady; or whether some directions, such as the north-west, were associated with higher velocities than others.

On the other hand, instruments not only give precision to the general impressions derived from the senses, which alone a savage can receive; but also enable us to discover some changes which our perceptions alone could never detect. For instance, by measuring heat-curves we can calculate the ordinary amount of daily range, and compare the value in London with that in Berlin, or New York; and we can also draw deductions from certain bends in the temperature-curve which would never have entered into the head of semi-civilized man. In like manner, there is in England a small increase of the windvelocity about 1 a.m. which has some scientific interest, but which certainly would not have been discovered without instrumental appliances.

But, in addition, the invention of the barometer has

given us another sense that is to say, the appreciation of the varying weight of the atmosphere, which was denied to our ancestors; and this book is the answer to the question how much weather-knowledge can be derived from observation of that instrument.

It will be found a distinguishing feature of this work that we have endeavoured to describe the weather in different shapes of isobars, so far as possible, in the language of popular prognostics. This language, while it contains many survivals of mythic speech, is still in current use, and gives a much more accurate picture of weather than more formal language. It is far more lifelike to talk of a cyclone-front as dirty and muggy than to report sky overcast, humidity ninety-eight per cent.; or to say that the sun "draws water" in straight isobars rather than c. 9 stratus (sky nine-tenths overcast, stratuscloud). We use, in fact, the phraseology of popular weather-lore to translate, as it were, the indications of instrumental readings into the language of common life.

At the same time, we have already examined most carefully the minuter fluctuations of some instrumental traces, and in various chapters we shall investigate the precise significance of the results of various arithmetical calculations which can be made from the numerical values derived from thermograms, etc.

The problems which the meteorologist has to solve are so complex and varied that he cannot afford to dispense with any possible assistance from whatever quarter; and our endeavour has been to convey to the reader the results of every line of investigation, and to collate the old and new meteorology into one compact science.

CHAPTER VI.

WIND AND CALM.

IN the preceding chapters we have only stated that in most cases the force or velocity of the wind is roughly proportional to the closeness of the isobars; but we shall now go into the details of the subject, and give the actual numbers which connect wind and gradients. We shall then point out various sources of variation which prevent us from laying down any law of wind with mathematical accuracy, and carry out the same idea with reference to the relation of the angle between the direction of the wind and the lie of the isobars. After that we shall extend these and other general principles of wind to the southern hemisphere, and conclude with a few general reflections on the subject.

GRADIENTS.

The relative closeness of any two isobars is not measured by the number of miles between them, but by the steepness of the barometric slope which they indicate. For instance, suppose that two isobars differ by 0-2 in. (5 mm.) of barometric level-say 29.7 and 29.9 in.

(755 and 760 mm.)-we do not measure their relative proximity by saying that they are thirty or ninety miles apart, but we think of the barometric slope with a rise of two-tenths of an inch (5 mm.) in either thirty or ninety miles. Then, to reduce this to a common standard, we take a uniform distance--in England fifteen nautical miles, or seventeen statute miles-and calculate how many hundredths of an inch the barometer would rise in those fifteen miles; that is to say, we treat the barometric slope like the slope of a hill, which is universally estimated by saying that the latter rises so many feet in a mile.

The slope between two isobars is called the barometric gradient, and, of course, it is measured square or at right angles to the isobars, just in the same way that we measure the slope of a hill between two contour lines.

For instance, suppose that in Fig. 36 the line ▲ B, drawn square to the isobars, is thirty nautical miles long, and that the isobars denote differences of two-tenths or twenty-hundredths of an inch; then the rise in fifteen nautical miles would be ten-hundredths of an inch; and we should say that there was a gradient of ten between the two stations A and B. If the distance between the same two isobars at C and D was ninety miles, the gradient over an observer at E would only be 0.2 × 100 × 15 = 3·3; and this last number would be the required gradient.

In practice we too often come across the error of taking the difference of pressure at two places, F and G, and calculating the gradient from the distance in miles between them. This always gives a smaller gradient than the real one, for the line of a gradient is always the shortest