Part 4

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 If the fall be slow. continuance

A violent storm from the If the mercury falls suddenly while the N.W. or N. wind is due W.

N.W., N., or N.E. winds, or If the mercury having been at its usual less wind, or less rain, or height, 29·95, is steady or rising, less snow _while the thermometer falls_ and the air becomes drier.

Wind and rain from S.E., S., If the mercury falls, while the and S.W. thermometer rises and the air becomes damp.

A violent storm from N.W., When the mercury falls suddenly with a N., or N.E. W. wind.

Snow If the mercury falls when the thermometer is low.

Less wind, or a change to When the mercury rises, after having N., or less wet been some time below its average height.

Strong wind or heavy squalls With the _first_ rise of the mercury from N.W., N., or N.E. 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 With a rapid and considerable fall of snow) the mercury.

Threatening, unsettled With an alternate rising and falling of weather the mercury.

Much wind, rain, hail, or When the mercury falls considerably. If snow, with or without the thermometer be low (for the season) lightning 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 | (including from South-east, by the the _North_, to the Eastward), | _South_, to the Westward), for dry, or less wet weather,--for | for wet weather,--for stronger less wind,--or for more than one | wind,--or for more than one of of these changes:-- | these changes:-- | EXCEPT on a few occasions when | EXCEPT on a few occasions when rain, hail, or snow comes from the | _moderate_ wind with rain (or Northward with _strong_ wind. | snow) comes from the Northward. -------------------------- For change of wind toward | For change of wind toward Northerly directions,-- | 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 | Add one-tenth of an inch to the contractions may be useful:-- | observed height for each hundred | feet the Barometer is above the RISE | FALL | half-tide level. | | FOR | FOR | The _average_ height of the | | Barometer, in England, at the NORTH | SOUTH | sea-level, is about 29·94 inches; | | and the _average_ temperature N.W.--N.--E. | S.E.--S.--W. | of air is nearly 50 degrees (London | | latitude). DRY | WET | | | The Thermometer falls about OR | OR | one degree for each three hundred | | feet of elevation from the ground, LESS | MORE | but varies with wind. | | ------ 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 | ------ | | Long foretold--long last, NORTH. | NORTH. | 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_.

In our climate the air is never completely dry, nor completely saturated with moisture, and the amount of aqueous vapour held in suspension is very variable. This fact has important bearings on many branches of industry, as also on the hygienic qualities of the atmosphere. The consideration that a certain amount of moisture in the air is necessary to the continuance of health will suggest the importance of maintaining that due proportion in the atmosphere of sick rooms, where the artificial heat so injudiciously used, often disturbs the healthful hygrometric condition of the air. Mr. GLAISHER is of opinion that the medical profession should enforce, as far as lies in their power, the use of this simple and effective instrument, which gives indications so important to the comfort of the patient.

The _amount_ of moisture in the air is _estimated_ by the use of instruments called Hygrometers, which may be thus classified:—

1. Hygrometers of Absorption.—Made with hair, oatbeard, catgut, seaweed, grass, chloride of calcium.

2. Hygrometers of Condensation.—Regnault’s, Daniell’s, Leslie’s, Dyne’s.

3. Hygrometers of Evaporation.—Mason’s Psychrometer, or Wet and Dry Bulb Thermometers.

By an ingenious application of the affinity of the oatbeard for moisture, Damp Detectors are constructed for tourists, commercial travellers, &c., to test moisture and avoid the consequences of sleeping in damp beds. They are strongly gilt, and resemble in size and shape a lady’s watch.

In Saussaure’s Hygrometer the frame is of brass, and the scale of the same metal silvered. It has an attached thermometer, and the indications are the result of the contraction and expansion of a prepared human hair, consequent upon its absorbing or yielding moisture. The scale is divided on the arc of a circle, and an index needle, working on an enlarged arc, multiples the indications.

Regnault’s Hygrometer (Fig. 39) consists of a thin and highly polished silver tube or bottle, into the neck of which is inserted a delicate thermometer. The bottle has a lateral tubular opening, to which is attached a flexible tube with an ivory mouthpiece.

Ether is poured into the silver tube in sufficient quantity to cover the bulb of the thermometer. The ether is then agitated by breathing through the flexible tube until, by the rapid evaporation thus produced, a condensation of moisture takes place, readily observable on the bright polished silver surface, and the temperature indicated by the thermometer at that moment is the dew-point.

Daniell’s Hygrometer, or Dew-point Thermometer (Fig. 40), consists of a glass tube, bent twice at right angles, each extremity terminating in a bulb about 1-1/2 inch in diameter, supported on a brass stand, to which a thermometer is attached to indicate the temperature of the surrounding air. The lower bulb is of blackened glass, to facilitate the observation of the dew-point; it is about three parts filled with pure ether, and contains a very delicate thermometer. The upper bulb at the extremity of the short stem is transparent, but covered with thin muslin, upon which, when an observation is made, pure ether is slowly dropped. The evaporation rapidly lowers the temperature, until a moment arrives at which dew condenses on the black bulb. A quick eye is necessary to note _this_ and the temperature shown by the thermometer _simultaneously_, the latter showing the degree at which the atmosphere is saturated with moisture _at the time of observation_. To avoid error, it is usual to note the temperature at which the dew disappears, and take the mean of the two temperatures.

Dyne’s Hygrometer, for showing the dew-point by direct observation, by means of iced water and black glass, enables the observer to dispense with the use of ether, and shows the dew-point with great distinctness.

The hygrometer in most general use is the wet and dry bulb thermometer, and for which Mr. Glaisher has calculated an elaborate set of tables, a brief abstract of which sufficient for general purposes is subjoined.

For finding the Degree of Humidity of the Air from Observations of a Dry Bulb and a Wet Bulb Thermometer, sometimes called Mason’s Psychrometer.

+-------------+-----------------------------------------+ | |DIFFERENCE BETWEEN DRY BULB AND WET BULB | | | READINGS. | |TEMPERATURE +------+------+------+------+------+------+ | BY THE | | | | | | | | DRY BULB | 2° | 4° | 6° | 8° | 10° | 12° | |THERMOMETER. | | | | | | | | +------+------+------+------+------+------+ | | DEGREE OF HUMIDITY. | +-------------+------+------+------+------+------+------+ | 34° | 79 | 63 | 50 | ... | ... | ... | | 36 | 82 | 66 | 53 | ... | ... | ... | | 38 | 83 | 68 | 56 | 45 | ... | ... | | 40 | 84 | 70 | 58 | 47 | ... | ... | | 42 | 84 | 71 | 59 | 49 | ... | ... | | 44 | 85 | 72 | 60 | 50 | ... | ... | | 46 | 86 | 73 | 61 | 51 | ... | ... | | 48 | 86 | 73 | 62 | 52 | 44 | ... | | 50 | 86 | 74 | 63 | 53 | 45 | ... | | 52 | 86 | 74 | 64 | 54 | 46 | ... | | 54 | 86 | 74 | 64 | 55 | 47 | ... | | 56 | 87 | 75 | 65 | 56 | 48 | ... | | 58 | 87 | 76 | 66 | 57 | 49 | ... | | 60 | 88 | 76 | 66 | 58 | 50 | 43 | | 62 | 88 | 77 | 67 | 58 | 50 | 44 | | 64 | 88 | 77 | 67 | 59 | 51 | 45 | | 66 | 88 | 78 | 68 | 60 | 52 | 45 | | 68 | 88 | 78 | 68 | 60 | 52 | 46 | | 70 | 88 | 78 | 69 | 61 | 53 | 47 | | 72 | 89 | 79 | 69 | 61 | 54 | 48 | | 74 | 89 | 79 | 70 | 62 | 55 | 48 | | 76 | 89 | 79 | 71 | 63 | 55 | 49 | | 78 | 89 | 79 | 71 | 63 | 56 | 50 | | 80 | 90 | 80 | 71 | 63 | 56 | 50 | | 82 | 90 | 80 | 72 | 64 | 57 | 51 | | 84 | 90 | 80 | 72 | 64 | 57 | 51 | | 86 | 90 | 80 | 72 | 64 | 58 | 52 | +-------------+------+------+------+------+------+------+

The total quantity of aqueous vapour which at any temperature can be diffused in the air being represented by 100, the percentage of vapour actually present will be found in the table _opposite_ the temperature of the dry thermometer, and _under_ the difference between the dry bulb and wet bulb temperatures. The degree of humidity for intermediate temperatures and differences to those given in the table can be easily estimated. Thus dry bulb 51°, wet bulb 46°, give 69 for the degree of humidity.

The instrument, as shown at page 48, consists of two thermometers attached to a support, which may be either slate or wood. The bulb of one of the thermometers has some thin muslin tied over it, and is kept moist by the capillary action of a thread dipping into a cistern of water placed underneath. It will be obvious that the amount of evaporation will be in proportion to the dryness of the air, and that the differences of temperature indicated by the two thermometers will be greatest when the atmosphere is dry, and least when the air is damp.

HYGROMETER PRECAUTIONS.

Hygrometers should be exposed in the shade free from air-currents. The covering of the wet bulb must be very thin. The supply of water must be carefully regulated. The bulb must be constantly _moist_, yet not _too wet_. The supply of water must be ample in dry weather. In damp weather water must not drip from the wet bulb. Water reservoir should be as far as possible from the dry bulb. Dry bulb must never receive moisture from any source. Use distilled, rain, or softest water procurable, for wet bulb. When lime deposits from use of hard water change muslin and worsted. Replenish reservoir after, or long before, taking an observation. Well wash muslin and worsted before using. Also wash occasionally while in use. Change muslin twice a month or according to condition. Dust and blacks must not be allowed to accumulate on muslin.

⎧ When wet bulb is frozen, wet with ice-cold water by brush. ⎪ The water will first freeze, then cool to air-temperature. ⎨ After which wet bulb falls a trifle lower than dry one. ⎩ Then temperature of evaporation may be noted.

⎧ In thick fog wet bulb reads _above_ dry bulb. ⎪ In cold calm weather, wet bulb reads _above_ dry bulb. ⎨ This is owing to the air being perfectly saturated. ⎪ Covered bulb cannot therefore show temperature as well as uncovered. ⎩ In such cases both readings are assumed to be identical.

It is important that the instrument should be protected not only from the sun’s direct rays, from rain and snow, but also from wind, the currents of which would, by increasing evaporation, cause the wet bulb thermometer to indicate a temperature not strictly due to the hygrometric condition of the atmosphere. For this purpose Thermometer Screens are employed. Illustrations of two forms are shown at Figs. 42 and 43; they should be placed facing the north at a distance of four feet from the ground. Fig. 42 shows the form adopted by the Board of Trade, for marine service, while Fig. 43 shows Mr. Stevenson’s double-louvred screen with perforated bottom, which ensures free ingress and egress of air, the exclusion of snow and rain, and the direct rays of the sun. Professor Wild recommends overlapping segments of sheet zinc for the construction of these screens, as possessing the advantage over wood of becoming sooner in _thermic equilibrium_ with the surrounding air, and thus preventing radiation. Stevenson’s Screen should be erected on legs four feet high, and should stand over grass on open ground. It should not be under the shadow of trees, nor within twenty feet of any wall.

CLOUDS.

The important office performed by clouds in the economy of nature entitles them to extended consideration. A cloud may be defined as “water-dust,” since aqueous vapour diffused through the air is invisible until the temperature is sufficiently lowered to produce condensation; no satisfactory explanation, however, has yet been given of the mode of suspension of this water-dust, nor why it remains suspended in opposition to gravitation. It is tolerably certain that electricity is not without its influence, though the apparently _stationary_ character of some clouds is deceptive, for while there may be no apparent motion in the mass the particles constituting the mass are undergoing continuous renewal, which justifies the assertion of Espy that every cloud is either a forming or dissolving cloud. Aeronauts in ascending from the earth pass through many successive alternations of cloud-strata and clear air which owe their existence to the varying temperature and degrees of humidity of the atmospheric currents so superposed.

Luke Howard in his Askesian Lectures, 1802, divides clouds into three primary modifications: cumulus, stratus, and cirrus, with intermediate forms resulting from combinations of the primaries, viz., cirro-cumulus, cirro-stratus, cumulo-stratus, and cumulo-cirro-stratus or nimbus. This nomenclature is now universally adopted.

CIRRUS, or mare’s tail cloud, appears as parallel, flexuous, or diverging streaks or fibres, partly straight. It is the lightest and the highest of all clouds, being seldom less than three miles, and often ten miles, above the earth, and shows the greatest variety of form. On account of its great height it is assumed to consist of minute snowflakes or crystals of ice, the refractions and reflections from which produce the halos, coronæ, and mock suns and moons which occur chiefly in this cloud and its derivatives. It retains its varied outlines longer than any other cloud; at sunrise it is the first to welcome the sun’s rays, and at sunset the last to part with them. It is the most useful of all clouds for weather warnings.

1. _Serene, settled weather may be expected_ when groups and threads of cirri are seen during a gentle wind after severe weather.

2. _A change to wet may be expected_ when, after continued fair weather, filaments, or bands of cirri (_apparently_ stationary), with converging ends, travel across the sky.

3. _Rain or snow, and windy, variable weather may be expected_ when cirri with fine tails vary much in a few hours.

4. _Continued wet weather may be undoubtedly expected_ when horizontal sheets of cirri fall quickly and pass into the cirro-stratus.

5. _A storm of wind and rain may be expected within forty-eight hours_ when fine threads of cirri seem brushed backward from the south-west.

CUMULUS.—This modification of cloud is most frequently seen on bright summer days, and is appropriately called “the day cloud” and “the summer cloud.” It is formed only in the daytime, in summer calms, and results from the rise of vapours from rivers, lakes, and marshes into the colder regions of the air, the lower portions of which are readily saturable. They are characterized by a horizontal base, from which they rise in dense conical and hemispherical masses rivalling mountains in their magnitude.

Their formation is due to the convection of heat from the earth’s surface, which renders the lower atmospheric strata capable of holding a larger amount of aqueous vapour and simultaneously establishes an upward current, which reaching the colder regions of the air brings about the condensation of the aqueous vapour into the elegant and ever-beautiful forms admired alike “by saint, by savage, and by sage.” These begin as mere specks, which enlarge until the sky is nearly covered in the afternoon, and towards sunset they generally disappear, their tops becoming cirri when the air is dry.

1. _Fine, calm, warm weather may be expected_ when cumuli are of moderate size and of pleasing form and colour.

2. _Cold, tempestuous, rainy weather may be expected_ when cumuli cover the sky, rolling over each other in dense, dark, and abrupt masses.

3. _Thunder may be expected_ when cumuli of hemispherical form are characterized by an extreme silvery whiteness.

4. _Rain may be expected_ when cumuli increase in number towards evening, sinking at the same time into the lower portions of the air.