The velocity pencil has only one turn on the cylinder, and its pitch is equal to a scale of fifty miles upon the paper. The direction pencil has likewise one turn on its cylinder, its pitch being equal to a scale of the cardinal points of the compa.s.s upon the paper.

The clock gives a uniform motion of half an inch per hour to the cylinder upon which the paper is fastened.

The registering mechanism of the instrument is very compact, requiring only a s.p.a.ce of about 18 inches by 8 inches.

In the Report of the British a.s.sociation for 1858, Mr. Beckley has given a detailed description of his anemometer, with drawings of all the parts.

=129. Self-Registering Lind's Anemometer.=--A Lind's wind-gauge, designed to register the maximum pressure, was exhibited at the International Exhibition 1862, by Mr. E. G. Wood. The bend of the syphon is contracted to obtain steadiness. On the leeward limb a hole is drilled corresponding in size with the contracted portion of the tube. The edge of the hole corresponds with the zero of the scale. On the pressure of the wind increasing, as much of the water as would have risen above the aperture flows away, and therefore the quant.i.ty left indicates the greatest pressure of the wind since the last setting of the instrument, which is done by filling it with water up to the zero point.

=130. Anemometric Observations.=--To ill.u.s.trate the value of anemometric observations, we quote from a paper by Mr. Hartnup, on the results obtained from Osler's Anemometer, at the Liverpool Observatory. The six years' observations, ending 1857, gave for the yearly average of the winds: North-easterly, on 60 days, at 78 miles per hour; North-westerly, on 112 days, at 154 miles per hour; South-easterly, on 115 days, at 110 miles per hour; South-westerly, on 77 days, at 138 miles per hour; and one day calm. From the same observations, the average variation in the strength of the wind during the 24 hours is:--11 miles per hour, the minimum force, occurring at 1-1/2 a.m.; until 6 a.m. it remains much the same, being then 113 miles per hour; at 10 a.m. it is 134 miles per hour; at 1-1/2 p.m. the wind is at its maximum strength, being 148 miles per hour; at 5 p.m. it is again 134 miles per hour, and at 9 p.m. 113 miles per hour. 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.

"There is evidence," says Admiral FitzRoy, "in Mr. Hartnup's very valuable anemometrical results, which seems to prove that to his observatory, in a valley, with buildings and hills to the north-eastward, the real polar current does not blow from N.E., but nearer S.E. By his reliable digest of winds experienced there, it appears that those most prevalent were from W.N.W. and S.S.E. But in England, generally, the _prevailing_ winds are _believed_ to be westerly, inclining to south-westerly, and north-easterly; while of all winds, the south-easterly is about the rarest.

"At Lord Wrottesley's observatory, in Staffordshire, about 530 feet above the sea, there appears to be considerably less strength of wind at any given time, when a gale is blowing _generally_, than occurs simultaneously at places along the sea-coast: whence the inference is, that undulations of the land's surface and hills, diminish the strength of wind materially by frictional resistance.

"All the synoptic charts. .h.i.therto advanced at the Board of Trade exhibit a marked diminution of force inland compared with that on the sea-coast.

Indeed, the coast itself offers similar evidence, in its stunted, sloping trees, and comparative barrenness."[14]

CHAPTER XIV.

INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.

=131. Atmospheric Electroscope.=--The simplest instrument for ascertaining at any time the electric condition of the atmosphere is an electroscope composed of two equal pieces of gold leaf, suspended from a bra.s.s support, and insulated, as well as protected from the movement of the air, by a gla.s.s covering. Fig. 90 represents such an instrument. The cap of the bra.s.s support is fitted for the reception, in the vertical direction, of a metallic rod, not less than two or three feet in length. The top of the rod carries a clip. The instrument acts according to the law, that bodies similarly electrified repel each other; but when dissimilarly electrified, they attract each other. To make an observation, the instrument is placed in the open air, and a lighted piece of cigar fusee, or touch-paper, is fixed in the clip. The electricity of the air is collected by the substance undergoing combustion, and conducted by the rod to the gold leaf; and the pieces, being similarly electrified, separate more or less according to the amount of electricity present. The kind is determined by the effect of either an excited stick of sealing-wax or rod of gla.s.s upon the electrified gold leaf. A rod of gla.s.s, when rubbed briskly with a silk handkerchief or piece of woollen cloth, becomes positively electrified, or excited, as it is termed. A stick of sealing-wax, similarly treated, acquires the negative state. If, therefore, an excited gla.s.s 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 a.n.a.logous to that of the gla.s.s, 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.

[Ill.u.s.tration: Fig. 90.]

By means of this very simple instrument, meteorological observers can readily ascertain the electric condition of the lower air at any time.

NOTE.--A book containing strips of gold leaf is sent with the Electrometer to replace the gold leaves when torn or broken in use.

To mount fresh gold leaves, unscrew the bra.s.s plate to which is attached the rod supporting the leaves; then moisten with the breath the flat piece of bra.s.s, and press it gently down on one strip of gold, whilst the book is only partly opened; the second leaf is attached in the same manner.

=132. Volta's Electrometer= is similar to the instrument just described, except that instead of gold leaf two light pieces of straw, or two pith b.a.l.l.s, are freely suspended from the conductor; the amount of the electric charge being estimated from the degrees of divergence, shown by a graduated arc.

=133. Peltier's Electrometer= is a much superior instrument in point of sensibility. A tall gla.s.s tube an inch or more in diameter, is connected to a gla.s.s receiver, mounted on a base fitted with levelling screws. At the top of the tube is formed a globe from four to five inches in diameter, which is thickly gilt on the exterior, so as to form a good conducting surface. A wire pa.s.ses from the ball down the tube into the receiver, where it is bent up, and ends in a steel point over the centre of the base. A bent wire, carrying a small magnetic needle, is balanced on the steel point, so that the magnet, with the fine wire, arranges itself horizontally in the direction of the magnetic meridian. If any cloud or portion of air in the neighbourhood be in an electrical state, it will act by induction upon the gilt ball, and the needle will be deflected from its north and south direction.

A graduated circle indicates the number of degrees of the deflection, which will be greater or less according to the tension of the electricity.

To ascertain whether the electricity is positive or negative, a stick of sh.e.l.lac or gla.s.s must be employed, as already described.

=134. Bohnenberger's Electroscope= may be fitted with a metallic conductor, and used with great advantage for observing atmospheric electricity. "The princ.i.p.al parts of the instrument, as improved by Becquerel, are the following:--_A B_, fig. 91, is a small dry galvanic pile of from 500 to 800 pairs, about a quarter of an inch in diameter; when the plates are pressed together, such a pile will be from 2 to 2-1/2 inches in length. The wires, which are bent so as to stand above the pile, terminate in two plates, _P_ and _M_, which are the poles of the pile.

These plates, which are 2 inches by 1/2 an inch, are parallel and opposite to each other. It is convenient for their opposite sides to be slightly convex, for them to be gilded or coated with platinum, and for them to run on the polar wires, by the latter being made to pa.s.s through a small hole in them. One of these plates will always be in a state of positive, and the other of negative, electricity; between them suspend the very fine gold leaf, _D G_, which is attached to the conductor, _C D_, of copper wire. If the leaf hang exactly between the two plates, it is equally attracted by each, and will therefore be in a state of repose. The apparatus should be protected by a bell-gla.s.s, fitting exactly, and having an opening at the top through which the copper wire, _C D_, pa.s.ses; the wire, however, is insulated by its being contained in a gla.s.s tube, which is made to adhere to the bell-gla.s.s by means of a small portion of sh.e.l.lac or gum-lac. Screw on a metal ball or plate, to impart to it the electricity you wish to test, which will be conveyed by the copper wire to the gold leaf, and the latter will immediately move towards the plate which has the opposite polarity. This electroscope is, beyond doubt, one of the most delicate ever constructed, and is well adapted to show small quant.i.ties of positive and negative electricity.

[Ill.u.s.tration: Fig. 91.]

"To ensure the susceptibility of electroscopes and electrometers placed under bell-gla.s.ses, precautions should be taken to render the air they contain as dry as possible, which may be effected by enclosing in a suitable vessel a little melted chloride of calcium beneath the gla.s.s."

The galvanic pile employed in this electroscope is that invented by Zamboni. "It differs from the common hydro-electric batteries princ.i.p.ally in this, that the presence of the electromotive liquid is dispensed with, and that in its place is subst.i.tuted some moist substance of low conducting power, generally paper. The electromotors in these piles are composed for the most part of Dutch gold (copper) and silver (zinc) paper pressed one on the other, with their paper sides together, out of which discs are cut with a diameter of from a quarter of an inch to an inch.

More powerful pairs of plates may be obtained by using only the silver paper and smearing its paper side with a thin coat of honey, on which some finely pulverized peroxide of manganese has been sprinkled, and all the sides similarly coated are presented one way. Powerful pairs of plates may also be made by pasting pure gold leaf on the paper side of zinc-paper.

These plates are then to be arranged, just as in the ordinary voltaic pile, one above the other, so that the similar metallic surfaces may all lie one way; press them tightly together; tie them with pretty stout silk threads, and press them into a gla.s.s tube of convenient size. The metal rims of the tubes, which must be well connected with the outermost pairs of plates, form the poles of the pile, the negative pole being in the extreme zinc surface, and the positive in the extreme copper or manganese surface.

"The electromotive energy called into action in these dry piles is less than that excited in the moist or hydro-electric piles, princ.i.p.ally on account of the imperfect conduction of the paper. The acc.u.mulation of electricity at their poles also goes on less rapidly, and consequently the electrical tension continues for a long while unaltered; whereas, in all moist piles, even in the most constant of them, the tension is maintained, comparatively speaking, for but a short time, on account of the chemical action and decomposition of the electromotive fluid--causes of disturbance which do not exist in the dry pile."[15]

=135. Thomson's Electrometer.=--Professor W. Thomson, of Glasgow, has devised an atmospheric electrometer, which is likely to become eminently successful, in the hands of skilful observers. It is mainly a torsion balance combined with a Leyden-jar. The index is an aluminium needle strung on a fine platinum wire, pa.s.sing through its centre of gravity, and stretched firmly between two points. The needle and wire are carefully insulated from the greater part of the instrument, but are in metallic communication with two small plates fixed beside the two ends of the needle, and termed the repelling plates. A second pair of larger plates face the repelling plates, on the opposite side of the needle, but considerably farther from it. These plates are in connection with the inner coating of a Leyden-jar, and are termed the attracting plates. The whole instrument is enclosed in a metal cage, to protect the gla.s.s Leyden-jar and the delicate needle.

The Leyden-jar should be charged when the instrument is used. Its effect is two-fold: it increases greatly the sensibility of the instrument, and enables the observer to distinguish between positive and negative electrification.

The air inside the jar is kept dry by pumice-stone, slightly moistened with sulphuric acid; by which means very perfect insulation is maintained.

Electrodes, or terminals, are brought outside the instrument, by which the Leyden-jar can be charged, and the needle system connected with the body, the electric state of which is to be tested.

For the purpose of testing the electric state of the atmosphere, the instrument is provided with a conductor and support for a burning match, or, preferably, with an arrangement termed a water-dropping collector; by either of which means the electricity of the air is conveyed to the needle system.

The needle abuts upon the repelling plates when not influenced by electricity, in which position it is at zero. It can always be brought back to zero by a torsion-head, turning one end of the platinum wire, but insulated from it, and provided with a graduated circle, so that the magnitude of the arc, that the torsion-head is moved through to bring the needle to zero, measures the force tending to deflect it.

The action of the instrument is as follows:--The Leyden-jar is to be highly charged, say negatively; and the repelling plates are to be connected with the earth. The needle will then be deflected against a stop, under the combined influence of attraction from the Leyden-jar, or attracting plates, and repulsion from the repelling plates due to the positive charge induced on the needle and its plates by the Leyden-jar plates. The platinum wire must then be turned round by the torsion-head so as to bring back the needle to zero; and the number of degrees of torsion required will measure the force with which the needle is attracted. Next, let the needle plates be disconnected from the earth, and connected with the insulated body, the electric state of which is to be tested. In testing the atmosphere, the conductor and lighted match, or water-dropping apparatus, must be applied.

If the electricity of the body be positive, it will augment the positive charge in the needle plates, induced by the Leyden-jar plates; and consequently the needle will be more deflected than by the action of the jar alone. If the electricity of the body be negative, it will tend to neutralize the positive charge; and the needle will be less deflected.

Hence the kind of electricity present in the air becomes at once apparent, without the necessity of an experimental test. The platinum wire must then be turned till the needle is brought to zero, and the number of degrees observed; which is a measure of the intensity of the electrification.

Any loss of charge from the Leyden-jar which may from time to time occur, reducing the sensibility inconveniently, may be made good by additions from a small electrophorus which accompanies the instrument.[16]

The instrument may be made self-recording by the aid of clockwork and photography. To effect this, a clock gives motion to a cylinder, upon which photographic paper is mounted. The needle of the electrometer is made to carry a small reflector; and rays from a properly adjusted source of light are thrown by the reflector, through a small opening, upon the photographic paper. It is evident, that as the cylinder revolves, a trace will be left upon the paper, showing the magnitude of, and variations in, the deflection of the needle.

=136. Fundamental Facts regarding Atmospheric Electricity.=--The _general_ electrical condition of the atmosphere is _positive_ in relation to the surface of the earth and ocean, becoming more and more positive as the alt.i.tude increases. When the sky is overcast, and the clouds are moving in different directions, it is subject to great and sudden variations, changing rapidly from positive to negative, and the reverse. 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 circ.u.mstances. It has an annual and a diurnal variation. There is a greater diurnal change of tension in winter than in summer. By comparing observations from month to month, a gradual increase of tension is perceived from July to February, and a decrease from February to July. The intensity seems to vary with the temperature. The diurnal variation exhibits two periods of greatest and two of least intensity. In summer, the _maxima_ occur about 10 a.m. and 10 p.m.; the _minima_ about 2 a.m. and noon. In winter, the _maxima_ take place near 10 a.m. and 8 p.m.; the _minima_ near 4 a.m. and 4 p.m.

The researches of Saussure, Beccaria, Crosse, Quetelet, Thompson, and FitzRoy have tended to show that during the prevalence of polar currents of air positive electricity is developed, and becomes more or less active according to the greater or less coldness and strength of wind; but with winds from the equatorial direction there is little evidence of sensitive electricity, and when observable, it is of the negative kind. Storms and gales of wind are generally attended, in places, with lightning and thunder; and as the former are very often attributed to the conflict of polar and equatorial winds, the difference of the electric tension of these winds may account for the latter phenomena. It is not our intention to enter upon the general consideration of thunderstorms; the facts which we have given may be of service to the young observer; and finally, as it is interesting to be able to judge of the locality of a thunderstorm, the following simple rule will be of service, and sufficiently accurate:--Note by a second's watch the number of seconds which elapse from the sight of the lightning to the commencement of the thunder; divide them by five, and the quotient will be the distance in miles. Thus, if thunder is heard ten seconds after the lightning was seen, the distance from the seat of the storm will be about two miles. The interval between the flash and the roll has seldom been observed greater than seventy-two seconds.

=137. Lightning Conductors.=--"The line of danger, whether from the burning or lifting power of lightning, is the line of strong and obstructed currents of air, of the greatest aerial friction."[17] Trees, church spires, wind-mills and other tall structures, obstruct the aerial currents, and hence their exposure to danger. The highest objects of the landscape, especially those that are nearest the thunder cloud, will receive the lightning stroke. The more elevated the object, the more likely is it to be struck. Of two or more objects, equally tall and near, the lightning is invariably found to select the best conductor of electricity, and even to make a circuitous path to get to it. Hence the application and evident advantage of metallic rods, called _lightning conductors_, attached to buildings and ships. A lightning conductor should be pointed at top, and extend some feet above the highest part of the edifice, or mast. It should be made of copper, which is a better conducting medium than iron, and more durable, being less corrosive. It must be unbroken throughout its length, and extend to the bottom of the building, and even some distance into the ground, so as to conduct the electricity into a well or moist soil. If it be connected with the lead and iron work in the structure of the house, it will be all the better, as affording a larger surface, and a readier means of exit for the fluid. In a ship, the lower end of the conductor should be led into communication with the hull, if of iron, and with the copper sheathing, if a wooden vessel; so that, spread over a large surface, it may escape more readily to the water.

=138. Precautions against Lightning.=--Experience seems to warrant the a.s.sumption that any building or ship, fitted with a substantial lightning conductor, is safe from danger during a thunderstorm. Should a house or vessel be undefended by a conductor, it may be advisable to adopt a few precautions against danger. In a house, the fire-place should be avoided, because the lightning may enter by the chimney, its sooty lining being a good conductor. "Through chimneys, lightning has a way into most houses; and therefore, it is wise, by opening doors or windows, to give it a way out. Wherever the aerial current is fiercest, there the danger is greatest; and if we kept out of the way of currents or draughts, we keep out of the way of the lightning."[18] Lightning evinces as it were a preference for metallic substances, and will fly from place to place, even out of the direct line of its pa.s.sage to the earth, to enter such bodies.

It is therefore well to avoid, as much as possible, gildings, silvered mirrors, and articles of metal. The best place is perhaps the middle of the room, unless a draught pa.s.ses, or a metallic lamp or chandelier should be hanging from the ceiling. The neighbourhood of bad conductors, such as gla.s.s windows, not being open, and on a thick bed of mattra.s.ses, are safe places. The quality of trees as lightning conductors is considered to depend upon their height and moisture, those which are taller and relatively more humid being struck in preference to their fellows; therefore, it is unwise to seek shelter under tall and wet trees during a thunderstorm. In the absence of any other shelter, it would be better to lie down on the ground.