“One may call a very simple diagram, a circle divided by a diameter from north-east to south-west, the _thermometer compass_. While the wind is shifting from south-west, by west, north-west, and _north to north-east_, the thermometer is falling, but while shifting from north-east, by east, south-east and south, towards _south and south-west_, the thermometer is rising. Now the barometric column does just the reverse. From north-east the barometer falls as the wind shifts through the east to south-east, south, and south-west, and from the south-west, as the wind shifts round northward to north-east, the barometer rises—it rises to west, north-west, north, and north-east. “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] [Footnote 14: The late Admiral Fitzroy.]