Enrico Angelo Lodovico Negretti

“ The observer, having his attention on the bulb and the thermometer, cannot always fix with precision the dew-point; and hence he is recommended to note the temperature at the appearance and at the disappearance of the dew, in order that the chance of error may be diminished. Without doubt, the necessarily long continuance of the observer near the instrument influences, to some extent, the observed temperatures; and the difficulty of not being always able to procure pure ether for the experiments is not the least of the drawbacks to the use of the instrument. ”

“ In practice, Fahrenheit determined the divisions of his scale from two fixed points, the freezing and boiling of water. The theory of the division, if we may so speak, was derived from the lowest cold observed in Iceland, and the expansions of a given portion of mercury” (Professor Trail) .
The divisions of the scale can be carried beyond the fixed points, if requisite, by equal graduations. Fahrenheit’s scale is very convenient in some respects. The meteorological observer is seldom troubled with negative signs, as the zero of the scale is much below freezing. ”

“ During fog, rain, hail, sleet, snow, and thunderstorms, the electrical state of the air undergoes many variations. The intensity of the electricity increases with hot weather following a series of wet days, or of wet weather coming after a continuance of dry days. The atmospheric electricity, in fact, seems to depend for its intensity and kind upon the direction and character of the prevailing wind, under ordinary circumstances. It has an annual and a diurnal variation. There is a greater diurnal change of tension in winter than in summer. ”

“ Not only is it of invaluable utility in connection with the barometer as a guide to the weather, but its indications are of service in showing the presence of a warm or cold current in the sea; many of the great oceanic currents being characterised by the warmth or coldness of their waters. In seas visited by icebergs, the habitual use of the thermometer would indicate their proximity, as the water is rendered colder for some distance around by the thawing of huge masses of ice. The water over a shoal in the sea is generally colder than the surface-water of the surrounding ocean ”

“ A steel index moves in the spirit in each limb of the syphon. The two indices are terminated at top and bottom with a bead of glass, to enable them to move with the least possible friction, and without causing separation of the spirit, or allowing mercury to pass easily. They would, from their weight, always rest upon the mercury; but each has a fine hair tied to its upper extremity and bent against the interior of the tube, which acts as a spring with sufficient elasticity to keep the index supported in the spirit in opposition to gravity. ”

“ Great clearness to light is perfectly compatible with great opacity to heat; the atmosphere may be charged with aqueous vapour while a deep blue sky is overhead; and on such occasions the terrestrial [Pg 85] radiation would, notwithstanding the ‘clearness,’ be intercepted.” The great range of the thermometer is attributable to the absence of that protection against gain or loss of heat which is afforded when aqueous vapour is present in the air; and during such weather the rapid abstraction of moisture from the surface of plants and animals is very deleterious to their healthy condition. ”

“ Below the contracted part is inserted a pipette—or Gay Lussac air-trap—which is a little elongated funnel with the point downwards. Its object is to arrest any air that may work in between the glass and the mercury. The bubble of air lodges at the shoulder, and can go up no farther. It is one of those simple contrivances which turn out remarkably useful. If any air gets into the tube, it does not get to the top, and therefore does not vitiate the performance of the barometer; for the mercury itself works up and down through the funnel. ”

“ It is a well-known fact that the barometer is as much, or even more affected by a change of wind as it is by rain; and the objection raised against a simple barometer reading, as leaving the observer in doubt whether to expect wind or rain, is removed by the addition of the Hygrometer, an instrument indicating the comparative degree of dryness or dampness of the air;—a most important item in the determination of the coming weather. ”

“ Of no other maximum thermometer can it be said that it is impossible to derange or put it out of order; hence, as regards durability, it surpasses all others. Nothing short of actual breakage can cause it to fail. Hence it is the most easily portable of all self-registering thermometers, an advantage which renders it suitable for travellers, and for transmission abroad. In the year 1852, the British Meteorological Society reported this thermometer to be “the best which has yet been constructed for maximum temperature, and particularly for sun observations.” ”

“ An increase of temperature produces greater, a diminution less elasticity in this air. The compensation for effects of temperature is adjusted by the process of “trial and error,” and only a few makers do it well. It is very often a mere sham. Admiral FitzRoy writes, in his Barometer Manual, “The known expansion and contraction of metals under varying temperatures, caused doubts as to the accuracy of the aneroid under such changes; but they were partly removed by introducing into the vacuum box a small portion of gas, as a compensation for the effects of heat or cold. ”

“ The mean temperature of the day is understood to be the average of twenty-four consecutive hourly readings of a thermometer; and meteorology now supplies formulæ whereby this result can be deduced from two or three observations only in a day. But we would observe that the actual mean temperature of any place has not such an important influence upon life, either animal or vegetable, as the abruptness and magnitude of the variations of temperature. Climate, therefore, should be estimated more by the range of the thermometer than by the average of its indications. ”

“ When mercury has been once made to pass the contraction, which nothing but the expansive force of heat can effect, and has risen in the tube, the upper end of the column registers the maximum temperature. To return the mercury to the bulb, we must apply a force equal to that which raised it in the tube; the force employed is gravity, assisted when necessary by a little agitation of the instrument. ”

“ The law connecting the pressure and the corresponding temperature of steam is the same as that upon which the boiling of fluids under diminished atmospheric pressure takes place. Hence, the indications of the thermometer become exponents of steam pressure. Engineers are furnished, in works on the steam-engine, with tables, from which the pressure corresponding to a given temperature, or the converse, can be obtained by mere inspection. ”

“ As rain is an important source of water-supply to rivers, canals, and reservoirs, it is evident that a knowledge of the probable fall for any season or month, at a given place, as furnished by averages of the observations of former years, will be the data upon which the engineer will base his plans for providing for floods or droughts; while the measurement of the actual quantity which has just fallen, as gathered from the indications of a series of gauges, will suggest to him the precautions to adopt either to economise or conduct away the in-pouring waters. ”

“ If, therefore, an excited glass rod be presented to the cap of the instrument, and it cause the pieces of gold leaf to diverge still further, the electric state of the air must be analogous to that of the glass, that is, positive; if they approach, it is negative. On the contrary, if a stick of sealing-wax be used, the pieces will be repelled more apart if they have acquired negative electricity from the air; and they will converge if they have a positive charge. ”

“ The descent of the coloured liquid in the other stem will mark the diminution of temperature caused by the evaporation of the water from the humid surface. The drier the ambient air is, the more rapidly will the evaporation go on; and the cold produced will be greater. When the air is nearly saturated with moisture, the evaporation goes on slowly; the cold produced is moderate, because the ball regains a large portion of its lost heat from surrounding bodies; and the degree of refrigeration of the ball is an index of the dryness of the air. ”

“ It is not desirable that the vernier should read nearer than one-hundredth of an inch. Two verniers and scales may be fitted one on either side of the mercurial column, so that one can denote the last reading, and thus show at a glance the extent of rise or fall in the interval. The scale and thermometer should be covered with plate glass. A cheap instrument has an open face and plain frame, with sliding vernier instead of rack-and-pinion motion. The barometer may or may not have a moveable bottom to the cistern, with screw for the purpose of securing the mercury for portability. ”

“ Rumford’s Differential Thermometer differs from that just described in simply containing only a small bubble of liquid, which lies in the centre of the tube, when both bulbs are similarly influenced. The bulbs and other parts of the tube contain air. When one bulb is more heated than the other, the bubble moves towards the one less heated; and the scale attached to the horizontal part of the tube affords a measurement of the difference of temperature. ”

“ CHAPTER XIII. APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION, PRESSURE, AND VELOCITY OF THE WIND.
Fig. 86.
122. The Vane.—The instrument by which the wind’s direction is most generally noted, is the vane, or weather-cock, and all that need be said of it here is that the points north, east, south and west, usually attached to it, should indicate the true and not the magnetic directions; and that care should be taken to prevent its setting fast. Very complicated instruments are required for ascertaining the pressure and velocity of the wind, and these are called Anemometers. ”

“ On the other hand, as cold produces condensation, any sudden fall of temperature causes the column of air over the locality to contract and sink to a lower level, whilst other air rushes in from above to supply the void; and, accordingly, the barometer rises. Should this air, as often happens, proceed from the north, it will contain in general but little moisture; and hence, on reaching a warmer latitude, will take up the vapour of the air, so that dry weather will result.
It is generally observed, that wind causes a fall in the instrument ”

“ The British Isles are more or less enveloped in fog, or mist, at the commencement of easterly winds, which, with a sudden change of wind, is exhibited even in summer; while the south-westerly winds, warm, and arriving from the ocean, deposit large quantities of rain by the cooling effect of the land, colder by reason of its latitude. When rain occurs with a northerly wind, it is probably due to the deposition from an upper south-westerly current, often apparently proved by the movements of the upper clouds. ”

“ Hence it appears that the wind falls to its minimum force much more gradually than it rises to its maximum; that the decrease and increase are equal and contrary, so that the curve is symmetrical; and that generally the force of wind is less at night than during the day. ”

“ If one end of the tube be hermetically closed, the other limb be cut off within a few inches of the bend, and the tube carefully filled with mercury; by placing it in a vertical position, the mercury will fall, if the closed limb be long enough, until it is about thirty inches higher than that in the exposed limb, where it will remain. Here the atmosphere presses upon the short column; but not upon the long one. It is this pressure, therefore, which maintains the difference of level. In fact, it forms a barometer without a cistern, the short limb answering the purpose of a cistern. ”

“ RULES FOR FORETELLING THE WEATHER.
A RISING BAROMETER.
A “Rapid” rise indicates unsettled weather.
A “Gradual” rise indicates settled weather.
A “Rise,” with dry air, and cold increasing in summer, indicates wind from northward; and if rain has fallen, better weather is to be expected.
A “Rise,” with moist air and a low temperature, indicates wind and rain from northward.
A “Rise,” with southerly wind, indicates fine weather.
A STEADY BAROMETER,
With dry air and a seasonable temperature, indicates a continuance of very fine weather. ”