Any persons ordering instruments of opticians may direct them to be previously forwarded to the observatory for verification.
A scale of charges is issued by the Committee which is exclusive of packing and carriage, or of rail expenses, when a special messenger is sent out. The Meteorological Office, Victoria Street, London, also receives and forwards instruments for verification to the Kew Observatory.
The Committee wish it to be understood that they cannot undertake the verification of an inferior class of instruments (such as barometers mounted upon wooden frames, and thermometers not graduated on the stem), and that the superintendent of the observatory may at his discretion decline to receive such instruments as he may consider unfit for scientific observation.
BAROMETER WARNINGS.
| May be Expected | |
| ----- | |
| Increasing storm | If mercury falls during a high wind from S.W., S.S.W., W. or S. |
| Violent but short | If the fall be rapid. |
| Less violent but of longer continuance | If the fall be slow. |
| A violent storm from the N.W. or N. | If the mercury falls suddenly while the wind is due W. |
| N.W., N., or N.E. winds, or less wind, or less rain, or less snow | If the mercury having been at its usual height, 29·95, is steady or rising, while the thermometer falls and the air becomes drier. |
| Wind and rain from S.E., S., and S.W. | If the mercury falls, while the thermometer rises and the air becomes damp. |
| A violent storm from N.W., N., or N.E. | When the mercury falls suddenly with a W. wind. |
| Snow | If the mercury falls when the thermometer is low. |
| Less wind, or a change to N., or less wet | When the mercury rises, after having been some time below its average height. |
| Strong wind or heavy squalls from N.W., N., or N.E. | With the first rise of the mercury after it has been very low (say 29). |
| Improved weather | When a gradual continuous rise of the mercury occurs with a falling thermometer. |
| Winds from S. or S.W. | If the mercury suddenly rising, the thermometer also rises. |
| Heavy gales from N. | Soon after the first rise of the mercury from a very low point. |
| Unsettled weather | With a rapid rise of the mercury. |
| Settled weather | With a slow rise of the mercury. |
| Very fine weather | With a continued steadiness of the mercury with dry air. |
| Stormy weather with rain (or snow) | With a rapid and considerable fall of the mercury. |
| Threatening, unsettled weather | With an alternate rising and falling of the mercury. |
| Much wind, rain, hail, or snow, with or without lightning | When the mercury falls considerably. If the thermometer be low (for the season) the wind will be N., if high, from S. |
| Lightning only | When the mercury is low, the storm being beyond the horizon. |
| Fine weather | With a rosy sky at sunset. |
| Wind and rain | When the sky has a sickly greenish hue. |
| Rain | When the clouds are of a dark Indian red. |
| Bad weather or much wind | When the sky is red in the morning. |
| EXPLANATORY CARD. | |||
| BY THE LATE VICE-ADMIRAL FITZROY, F.R.S., ETC. | |||
| WEATHER GLASSES. | |||
| The Barometer Rises | The Barometer Falls | ||
| for Northerly wind | for Southerly wind | ||
| (including from North-west, by the North, to the Eastward), | (including from South-east, by the South, to the Westward), | ||
| for dry, or less wet weather,—for less wind,—or for more than one of these changes:— | for wet weather,—for stronger wind,—or for more than one of these changes:— | ||
| Except on a few occasions when rain, hail, or snow comes from the Northward with strong wind. | Except on a few occasions when moderate wind with rain (or snow) comes from the Northward. | ||
| For change of wind toward Northerly directions,— | For change of wind toward Southerly directions,— | ||
| a thermometer falls. | a thermometer rises. | ||
| Moisture or dampness in the air (shown by a Hygrometer) increases before rain, fog, or dew. | |||
| On barometer scales the following contractions may be useful:— | Add one-tenth of an inch to the observed height for each hundred feet the Barometer is above the half-tide level. | ||
| RISE | FALL | ||
| FOR | FOR | The average height of the Barometer, in England, at the sea-level, is about 29·94 inches; and the average temperature of air is nearly 50 degrees (London latitude). | |
| NORTH | SOUTH | ||
| N.W.—N.—E. | S.E.—S.—W. | ||
| DRY | WET | The Thermometer falls about one degree for each three hundred feet of elevation from the ground, but varies with wind. | |
| OR | OR | ||
| LESS | MORE | ||
| WIND. | WIND. | “When the wind shifts against the sun, | |
| Trust it not, for back it will run.” | |||
| EXCEPT | EXCEPT | First rise after very low | |
| Indicates a stronger blow. | |||
| WET FROM | WET FROM | ||
| NORTH. | NORTH. | Long foretold—long last, | |
| Short notice—soon past. | |||
| (In South Latitude read South for North.) | |||
IV.—CONDENSATION.
Dew is a deposition of moisture from the air, resulting from the condensation of the aqueous vapour of the atmosphere on substances which have become cooled by the radiation of their heat. This is, in fact, the substance of Dr. Wells’s famous Theory of Dew, enunciated in 1814, and which, according to Dr. Tyndall, “has stood the test of all subsequent criticism, and is now universally accepted,” and by which all the phenomena of dew may be explained.
Dr. Wells’s experiments were interesting and conclusive. He exposed definite weights (10 grains) of wool to the air on clear nights, one on a four-legged stool, the other under it, the upper portion gained 14 grains in weight, the lower only 4 grains. On an evening when one portion of wool, protected by a curved pasteboard roof, gained only 2 grains, a similar portion on the top of the miniature roof gained 16 grains. A little reflection will suggest the explanation: radiation from the wool was arrested by the pasteboard cover, while the portion fully exposed to the sky lost all its heat, and thus condensation ensued. Dr. Wells speaks with such candour, and so pointedly, on this fact and its consequences, that his words may be advantageously quoted: “I had often, in the pride of half-knowledge, smiled at the means frequently employed by gardeners to protect tender plants from cold, as it appeared to me impossible that a thin mat, or any such flimsy substance, could prevent them from attaining the temperature of the atmosphere, by which alone I thought them liable to be injured. But when I had learned that bodies on the surface of the earth become during a still and serene night colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice I had before deemed useless.”
Familiar instances of the formation of dew will have been noted by many “watchers;” e. g., breathing on a cold pane of glass, a tumbler of cold water becoming dew-covered on being brought into a warm room, the outside of a tankard of iced claret cup, &c. When, radiation is so free and rapid that the temperature is below the freezing point, the dew freezes as it forms, producing hoar-frost.