“The effect of the wind thus shifting round when traced upon paper by a curve, seems certainly wave-like to the eye; but I believe it to be simply consequent on the wind shifting round the compass, and indicating alteration in the barometric column.
“If the wind remained north-east, say three weeks, there would be no wave at all—there would be almost a straight line along a diagram (varying only a little for strength). The atmospheric line, in such a case, remains at the same height, and the barometer remains at 30 inches and (say) some three or four-tenths, for weeks together. So likewise when the wind is south-westerly a long time, or near that point, the atmospheric line remains low, towards 29 inches. Thus, such ‘atmospheric waves’ may be an optical delusion.
“The diagram alluded to above shows how the barometer and thermometer may be used in connection with each other in foretelling wind, and consequently weather, that is coming on, because as the one rises, the other generally falls, and if you take the two together and confront with their indications the amount of moisture in the air at any time, you will scarcely be mistaken in knowing what kind of weather you are likely to have for the next two or three days, which for the gardener, the farmer, soldier, sailor, and traveller must be frequently of considerable importance.”[[14]]
[14]. The late Admiral Fitzroy.
We are indebted to M. Buys Ballot, a Dutch meteorologist, for an invaluable generalization, the importance of which it is almost impossible to over-estimate. This distinguished savant says:—“It is a fact above all doubt that the wind that comes is nearly at right angles to the line between the places of highest and lowest barometer readings. The wind has the place of lowest barometer at its left hand, and is stronger in proportion as the difference of barometer readings is greater.” These facts have been variously stated by other writers; for example: “Stand with your back to the wind, and the barometer will be lower on your left hand than on your right;” “Facing the wind the centre of depression bears in the right-hand direction,” statements which can be verified at any time by a brief study of the “Weather Charts” now published in the daily journals. The value of the law consists in its connecting the surface winds of our planet with the actual pressure of the air itself, and it admits of the following tabulation:—
| The wind is | The wind is | The wind is | The wind is | ||||||
| NORTHERLY | SOUTHERLY | EASTERLY | WESTERLY | ||||||
| when the | when the | when the | when the | ||||||
| BAROMETER | BAROMETER | BAROMETER | BAROMETER | ||||||
| is, in the | is, in the | is, in the | is, in the | ||||||
|
| N. High. | N. Low. | ||||||
| S. Low. | S. High. | ||||||||
|
| ||||||||
| E. Low. | E. High. | ||||||||
| W. High. | W. Low. |
which can be verified by the reader from the daily Weather Charts in the newspapers.
The above are deductions from Buys Ballot’s Law, still further impressed on the memory by taking four outline maps of the British Isles, inserting the names of Thurso, Penzance, Yarmouth, and Valentia, with barometer readings of the kind above named at each place, and then drawing a large arrow in red ink across the centre of each map in the direction appropriate to the readings.