Part 7

Dr. Robinson, of Armagh, introduced an instrument, in 1850, which consists of four hemispherical copper cups attached to the arms of a metal cross. The vertical axis upon which these are secured has at its lower extremity an endless screw placed in gear with a train of wheels and pinions. Each wheel is graduated respectively to 1/10th, 1 mile, 10 miles, 100 miles, 1,000 miles, and these revolve behind a fixed index, the readings of which are taken according to the indications on the dials.

Dr. Robinson entertained the theory that the cups (measuring from their centres) revolved with one-third of the wind’s velocity; and this theory having been fully supported by experiment, due allowance has been made in graduating the wheels so that the true velocity is obtained by direct observation.

In an improved form of this anemometer the hemispherical cups are retained, but the index portion of the instrument consists of two graduated concentric circles, the inner one representing five miles divided into 10ths, and the outer one bearing 100 divisions, each of which is equivalent to five miles. At the top of the dial is a fixed index, which, as the toothed wheel revolves, marks on the inner circle the miles (up to five) and 10ths of miles the wind has travelled, while a movable index, which revolves with the wheel, indicates on the outer circle the passage of every five miles.

This instrument can be made very portable by removing the arms bearing the cups, when the whole may be packed with iron shaft in a case 15 × 13 × 4 inches. It may be placed in any desired position by screwing the iron shaft supplied with it into the hole provided for the purpose, and fixing the apparatus on a pole or on an elevated stand, if possible, in an open space exposed to the _direct_ action of the wind.

If, when placing the instrument, the hands stand at 0, the next reading will, of course, show the number of miles the wind has traversed; but, should they stand otherwise, the reading may be noted and deducted from the second reading, thus: Suppose the fixed index points to 2·5 and the movable index to 125, the reading after 12 hours may be 200 on the outer circle and 3·0 on the inner circle: these added together yield 203. By deducting the previous reading 127·5, we have the true reading—viz., 75·5 miles as the distance travelled by the wind.

Having obtained the velocity of the wind in this manner in miles per hour, the table on page 83, from Col. Sir Henry James’s “Instructions for Taking Meteorological Observations,” will enable the observer to calculate the pressure in pounds per square foot.

WEATHER NOTATION.

The following letters are used to denote the state of the weather:—

_b_ denotes blue sky, whether with clear or slightly hazy atmosphere. _c_ „ cloudy, that is detached opening clouds. _d_ „ drizzling rain. _f_ „ fog. _h_ „ hail. _l_ „ lightning. _m_ „ misty, or hazy so as to interrupt the view. _o_ „ overcast, gloomy, dull. _p_ „ passing showers. _q_ „ squally. _r_ „ rain. _s_ „ snow. _t_ „ thunder. _u_ „ ugly, threatening appearance of sky. _v_ „ unusual visibility of distant objects. _w_ „ wet, that is dew.

A letter repeated denotes much, as _rr_, heavy rain; _ff_, dense fog; and a figure attached denotes duration in hours, as 14_r_, 14 hours’ rain.

By the combination of these letters all the ordinary phenomena of the weather may be recorded with certainty and brevity.

_Examples._—_bc_, blue sky with less proportion of cloud; _cb_, more cloudy than clear; 2_rrllt_, heavy rain for two hours, with much lightning, and some thunder.

VELOCITY AND PRESSURE OF THE WIND.

The Pressure varies as the Square of the Velocity, or _P_ ∝ _V_^2. The Square of the Velocity in Miles per Hour multiplied by ·500 gives the Pressure in lbs. per square Foot, or _V_^2 × ·005 = _P_. The Square Root of 200 times the Pressure equals the Velocity, or √(200 × _P_) = _V_.

The subjoined Table is calculated from this data, by COL. SIR HENRY JAMES, of the Ordnance Survey Office.

+-------------------------------------------------------------------+ |Pressure in | |lbs. per | |Square Foot. | | |Velocity in | | |Miles | | |per Hour. | | | |Pressure in | | | |lbs. per | | | |Square Foot. | | | | |Velocity in | | | | |Miles | | | | |per Hour. | | | | | |Pressure in | | | | | |lbs. per | | | | | |Square Foot. | | | | | | |Velocity in | | | | | | |Miles | | | | | | |per Hour. | | | | | | | |Pressure in | | | | | | | |lbs. per | | | | | | | |Square Foot. | | | | | | | | |Velocity in | | | | | | | | |Miles | | | | | | | | |per Hour. | | | | | | | | | |Pressure in | | | | | | | | | |lbs. per | | | | | | | | | |Square Foot. | | | | | | | | | | |Velocity | | | | | | | | | | |in Miles | | | | | | | | | | |per Hour.| +-----+------+-----+------+-----+------+-----+------+-----+---------+ | oz. | | lbs.| | lbs.| | lbs.| | lbs.| | | 0·08| 1·000| 6·75|36·742|17·75|59·581|28·75|75·828|39·75| 89·162 | | 0·25| 1·767| 7·00|37·416|18·00|60·000|29·00|76·157|40·00| 89·442 | | 0·50| 2·500| 7·25|38·078|18·25|60·415|29·25|76·485|40·25| 89·721 | | 0·75| 3·061| 7·50|38·729|18·50|60·827|29·50|76·811|40·50| 90·000 | | 1·00| 3·535| 7·75|39·370|18·75|61·237|29·75|77·136|40·75| 90·277 | | 2·00| 5·000| 8·00|40·000|19·00|61·644|30·00|77·459|41·00| 90·553 | | 3·00| 6·123| 8·25|40·620|19·25|62·048|30·25|77·781|41·25| 90·829 | | 4·00| 7·071| 8·50|41·231|19·50|62·449|30·50|78·102|41·50| 91·104 | | 5·00| 7·905| 8·75|41·833|19·75|62·819|30·75|78·421|41·75| 91·378 | | 6·00| 8·660| 9·00|42·426|20·00|63·245|31·00|78·740|42·00| 91·651 | | 7·00| 9·354| 9·25|43·011|20·25|63·639|31·25|79·056|42·25| 91·923 | | 8·00|10·000| 9·50|43·588|20·50|64·031|31·50|79·372|42·50| 92·195 | | 9·00|10·606| 9·75|44·158|20·75|64·420|31·75|79·686|42·75| 92·466 | |10·00|11·180|10·00|44·721|21·00|64·807|32·00|80·000|43·00| 92·736 | |11·00|11·726|10·25|45·276|21·25|65·192|32·25|80·311|43·25| 93·005 | |12·00|12·247|10·50|45·825|21·50|65·574|32·50|80·622|43·50| 93·273 | |13·00|12·747|10·75|46·368|21·75|65·954|32·75|80·932|43·75| 93·541 | |14·00|13·228|11·00|46·904|22·00|66·332|33·00|81·240|44·00| 93·808 | |15·00|13·693|11·25|47·434|22·25|66·708|33·25|81·547|44·25| 94·074 | | | |11·50|47·958|22·50|67·082|33·50|81·853|44·50| 94·339 | | lbs.| |11·75|48·476|22·75|67·453|33·75|82·158|44·75| 94·604 | | 1·00|14·142|12·00|48·989|23·00|67·823|34·00|82·462|45·00| 94·868 | | 1·25|15·811|12·25|49·497|23·25|68·190|34·25|82·764|45·26| 95·393 | | 1·50|17·320|12·50|50·000|23·50|68·556|34·50|83·066|45·50| 95·131 | | 1·75|18·708|12·75|50·497|23·75|68·920|34·75|83·366|45·75| 95·655 | | 2·00|20·000|13·00|50·990|24·00|69·282|35·00|83·666|46·00| 95·916 | | 2·25|21·213|13·25|51·478|24·25|69·641|35·25|83·964|46·25| 96·176 | | 2·50|22·360|13·50|51·961|24·50|70·000|35·50|84·261|46·50| 96·436 | | 2·75|23·452|13·75|52·440|24·75|70·356|35·75|84·567|46·75| 96·695 | | 3·00|24·494|14·00|52·915|25·00|70·710|36·00|84·852|47·00| 96·953 | | 3·25|25·495|14·25|53·385|25·25|71·063|36·25|85 146|47·25| 97·211 | | 3·50|26·457|14·50|53·851|25·50|71·414|36·50|85·440|47·50| 97·467 | | 3·75|27·386|14·75|54·313|25·75|71·763|36·75|85·732|47·75| 97·724 | | 4·00|28·284|15·00|54·772|26·00|72·111|37·00|86·023|48·00| 97·979 | | 4·25|29·154|15·25|55·226|26·25|72·456|37·25|86·313|48·25| 98·234 | | 4·50|30·000|15·50|55·677|26·50|72·801|37·50|86·602|48·50| 98·488 | | 4·75|30·822|15·75|56·124|26·75|73 143|37·75|86·890|48·75| 98·742 | | 5·00|31·622|16·00|56·568|27·00|73·484|38·00|87·177|49·00| 98·994 | | 5·25|32·403|16·25|57·008|27·25|73·824|38·25|87·464|49·25| 99·247 | | 5·50|33·166|16·50|57·415|27·50|74·161|38·50|87·749|49·50| 99·498 | | 5·75|33·911|16·75|57·879|27·75|74·498|38·75|88·034|49·75| 99·749 | | 6·00|34·641|17·00|58·309|28·00|74·833|39·00|88·317|50·00| 100·000 | | 6·25|35·355|17·25|58·736|28·25|75·166|39·25|88·600| | | | 6·50|36·055|17·50|59·160|28·50|75·498|39·50|88·881| | | +-----+------+-----+------+-----+------+-----+------+-----+---------+

This is the only table hitherto much in use for converting velocity into pressure, and was prepared by Smeaton and others. It does not, however, express the true relation, which has yet to be determined.

The Anemograph, or Self-Recording Wind Gauge, has for its object the registration of the velocity and direction of the wind from day to day. Figs. 59 and 60 show the form designed and arranged by Mr. Beckley, of the Kew Observatory, which has been adopted by the Meteorological Office.

It consists of a set of hemispherical cups and vanes, which are exposed on the roof of the house, and of the recording apparatus, which is placed inside the house.

The motion imparted to the hemispherical cups by the wind is communicated to the steel shaft B, which, passing through the hollow shaft C, and having at its lower end an endless screw, works into a series of wheels in the iron box D, which reduces the angular velocity 7,000 times. At the required distance the motion, having emerged at E, is connected with F, where, by means of bevelled wheels, it moves the spiral brass registering pencil C, which is arranged so that each revolution records 50 miles of velocity on the prepared paper H.

The direction of the wind is indicated by the arrow L, which is kept in position by the fans M. These communicate, by an endless screw and train of wheels, through the shaft C and the box D to the recording apparatus, consisting of a spiral brass pencil, which in one revolution records variations through the cardinal points of the compass, on the same prepared paper as that which receives the record of velocity.

The paper is held on the drum by two small clips, and may be readily changed, by unclamping the cross V, without disturbing the drum or any other part of the instrument.

VI.—ELECTRIFICATION.

William Gilbert, a physician of Colchester, first showed in 1600 that the earth as a whole has the properties of a magnet, and consequently that the directive action exerted by it upon a compass needle represents only a special case of the mutual action of two magnets. In 1845, Faraday established the fact that susceptibility to magnetic force is not, as was generally believed, confined to iron, nickel, and a few other substances, but is a property of all substances. According to Balfour Stewart, auroræ and earth currents may be regarded as secondary currents resulting from changes in the earth’s magnetism. Magnetic phenomena are included under the general term terrestrial magnetic elements, and consist of magnetic declination, inclination, and intensity.

These are for convenience determined separately; the first by an instrument called a _Declinometer_, and the second by an _Inclinometer_ or _Dipping Needle_. The Declinometer is also made to serve the additional purpose of measuring the _intensity_ of the earth’s magnetic force, which it effects on a principle similar to that by which the force of gravity is determined by the oscillations of a pendulum of known length on any given portion of the earth’s surface. The declinometer needle is made to oscillate, and the number of oscillations in a given time counted; due allowance being made for the strength of the needle, it is obvious that the force which restores the needle to rest can be estimated. To ascertain the angle of _declination_, the zero line of the compass card is made to coincide with the geographical north and south line; and the angle which the direction of the needle makes with this line is then read off on a graduated circle over which the needle turns. The magnetic _inclination_ or _dip of the needle_ is estimated by observing the inclination to a horizontal plane of a needle turning on the vertical plane which passes through the magnetic north and south points.

Fig. 62 shows a simple form of magnetic needle suspended on a fine steel point, which is supported by a brass stand; the addition of a graduated circle would constitute such an arrangement a Declinometer.

Fig. 63 gives the appearance of the dipping needle, or Inclinometer, and Fig. 64 an arrangement by which both kinds of terrestrial as well as local attraction may be shown.

These components of the earth’s magnetism undergo not only an annual but a daily and even hourly variation, apparently connected in some occult manner with the frequency of the sun’s spots. The needle sometimes suffers such exceptional perturbations as to suggest the idea of a magnetic storm. These disturbances are usually accompanied (in polar regions) by luminous phenomena called auroræ. Continuous automatic records of them, therefore, is of great value, as facilitating inductive research which may lead to valuable practical results.

Accordingly the Royal Society have adopted for the Kew and other observatories the form of Magnetograph, or Self-recording Magnetometer, shown at Fig. 61, by means of which the variations just referred to are registered by the oscillations of three magnets on photographically prepared paper, stretched on a drum revolved by clockwork.

One magnet is suspended in the magnetic meridian by a silk thread, and, by the aid of a mirror attached, it describes on the cylinder, moved by clockwork in the centre pier, all the variations in the magnetic _declination_.

The other two components of the magnetic force of the earth are given by the other magnets. That recording the vertical variations rests on two agate edges under a glass shade, while the horizontal component magnet is suspended by a double silk thread, under the shade to the right of the picture, being retained by the tension of the thread in a position nearly at right angles to the magnetic meridian.

The clock box in the centre covers the three revolving cylinders bearing the sensitive photographic paper, and to each magnet is attached a semicircular mirror, which reflects the rays from a gas jet to one of the cylinders, and thus describes by a curved line the oscillations of the magnet. A second semicircular mirror is _fixed_ to the pier on which the instrument stands, and consequently describes a straight line, or zero, from whence the curves are measured.

To avoid errors attending sudden changes of temperature, underground vaults are always chosen for magnetic observations, and also on account of light being more easily and perfectly excluded.

ATMOSPHERIC ELECTRICITY.

Since the performance of Franklin’s famous kite experiment, by which he determined the identity of lightning with the electrical discharge from a machine, much attention has been devoted, not only to that form of atmospheric electricity which displays itself in the thunder-cloud, but to the electric condition of the air in all states of the weather. These researches have established the fact that the air is always in an electrical condition, even when the sky is clear and free from thunder-clouds. The instruments employed for ascertaining the kind and intensity of atmospheric electricity are called Electroscopes. Fig. 65 shows a modification of Saussure’s Electroscope, the basis of which is a narrow-mouthed flint glass bottle with a divided scale to indicate the degree of divergence of the gold leaves or straws. To protect the lower part from rain, it is covered by a metallic shield about five inches in diameter. Bohnenberger’s Electroscope indicates the presence and quality of _feeble_ electric currents. Peltier’s Electrometer yields the same result by the deflection of a magnetic needle. This latter has been in use at Brussels for thirty years, and at Utrecht for twenty years, and is highly recommended.

Singer’s Atmospheric Electroscope is an efficient form of the instrument in which an ordinary gold-leaf electrometer has attached to its circular brass plate a brass rod two feet in length, with a clip at its upper extremity to receive a lighted paper or cigar fusee. The electricity of the air in immediate contact with the flame, causes, by induction, electricity of the opposite nature to accumulate at the upper extremity, where it is constantly carried off by the convection currents in the flame, leaving the conductor charged with the same kind and power of electricity as that contained in the air at the time of the experiment. The principle of this method was initiated by Volta, and has been extended and applied by Sir William Thomson in his Water-dropping Collector, which consists of an insulated cistern from which water escapes through a jet so fine that it breaks into drops immediately after leaving the nozzle of the tube. The result of this is that in half a minute from the starting of the stream the can is found to be electrified to the same extent as the air at the point of the tube. The scale value of each instrument has to be separately determined by repeated comparative experiments, and involves much delicacy of manipulation.

It is chiefly important for the ordinary observer to know that the occurrence of thunder and lightning should be always noted in the column headed “Remarks.”

The destructive effects of lightning are too well known to need description here; the means, however, by which these may be averted demand a brief notice. Lightning when discharged from a cloud will always choose the better of any two conductors which may present themselves. The _stone_ of a church steeple and the _wood_ of a ship’s mast are bad conductors, but a galvanized iron wire rope is the best possible conductor, and accordingly this material is now generally employed for the purpose. A lightning conductor consists of three parts: 1, the rod, which extends beyond the summit of the building, 2, the conductor, which connects the rod with the underground portion, and 3, the part underground. The connection between each of these must be absolutely perfect, or the conductor will be faulty. The top is usually of solid copper tipped with platinum (Fig. 66), the body of galvanized iron rope, so as to adapt itself to the inequalities of the building and yet have no sharp turns in it, while the part underground is of solid iron rod. This latter portion should extend straight underground for two feet, and being bent at right angles away from the wall, should rest in a horizontal drain 10 to 15 feet long filled with charcoal, and be again bent downwards into a well of water. Should water not be available, it should rest in the centre of a hole 15 feet deep and 10 inches in diameter, tightly packed with charcoal, which, while conducting the electricity from the rod into the earth, serves also to preserve the iron from rusting.

OZONE.

The atmosphere, besides holding the vapour of water diffused throughout its mass, contains also minute traces of carbonic acid and ammonia, and a very remarkable substance called Ozone. Oxygen, one of the component gases of the atmosphere, is capable of existing in two conditions; one in which it is comparatively passive, and another in which it possesses exceptional chemical activity, dependent apparently upon its electrical condition, and in which state it possesses a peculiar smell which has caused it to be named ozone.[16] The characteristic odour is always observable near a powerful electric machine when it is being worked, near a battery used for the decomposition of water, and in the air after the passage of a flash of lightning. Its presence is most marked near the sea-coast, and in localities remarkable for their salubrity; and on account of its influence on health, it has been proposed by Schonbein and others to include ozonometrical observations with the ordinary meteorological observations.

[Footnote 16: Greek _ozo_, I smell.]

Although in minute quantities it is favourable to health, when existing in undue proportion it irritates the mucous membrane of the nose and throat, producing painful sores. It attacks india-rubber, bleaches indigo, and oxidizes silver and mercury, differing in all these points from ordinary atmospheric oxygen.

The chemical energy it possesses (which exceeds that of ordinary oxygen as much as the latter exceeds atmospheric air as an oxidizing agent) affords the means of ascertaining its presence and quantity. It liberates iodine from its combination with potassium, and free iodine colours starch a deep blue.

Schonbein, the discoverer of ozone, found that when strips of paper previously saturated with starch and iodide of potassium and dried were exposed freely to the air but protected from rain and the direct action of the sun, they underwent a peculiar discoloration (when immersed in water) after an exposure of 24 hours. A scale of tints numbered from one to ten afforded the means of comparative observation, and thus the Ozonometer was constructed, and a means established of registering the amount of ozone in the air of various localities from day to day.

Schonbein also observed that the proportion of ozone was largely augmented after heavy falls of snow. For the exposure of the ozone papers, an ozone cage is employed, as shown at Fig. 67.

Ozone may be prepared artificially as a disinfectant by cautiously mixing without friction or concussion equal parts of peroxide of manganese, permanganate of potash, and oxalic acid. For a room containing 1,000 cubic feet, two teaspoonfuls of the powder, placed in a dish and moistened with water occasionally, will develop the ozone and disinfect the surrounding air without producing cough.

The most important and interesting series of facts, however, connected with ozone are those established by the researches of M. Houzeau, who states:—

1. That country air contains an odorous oxidizing substance, with the power of bleaching blue litmus, without previously reddening it, of destroying bad smells, and of bluing iodized red litmus.

2. That this substance is ozone.