METEOROLOGICAL.

Hokitika Guardian, 1 March 1924, Page 2

METEOROLOGICAL.

The above reading was taken at 12 noon to-day. The reading at 0 a.m. this morning was 30.2. The barometer is an instrument by which the weight or pressure of the atmosphere is estimated. Torricelli, in Kilo, constructed the first instrument. It had been found, in attempting to raise water from a very deep well near Florence, that, in spite ol all the pains taken in fitting the piston and valves, the water could by no effort be made to riso higher in the pump than about 32 feet, 'ibis phenomenon Torricelli accounted lor by attributing pressure to the air. He. reasoned that water will riso in a vacuum only to a certain height, so that the downward pressure or weight of the column of water will just balance the pressure of the atmosphere ; and ho further argued that if a fluid heavier than water be used it will not rise so high in the tube as water. Jo prove this, he selected a glass tube about a quarter of an inch in diameter and four feet, long, and hermetically sealed one of its ends; be then filled it with mercury, and, applying his linger to the open end, inverted it in a basin containing mercury. 'The mercury instantly sank to nearly 30 inches above the snrfaco of the mercury in the basin, leaving in the top of the tube an apparent vacuum, which is, indeed, one of the most perfect that can yet bo produced, and is called after this great experimenter, the "Torricellian vacuum.” Ho next converted tlio mercurial column into a form suited for observation by bending the lower end of the tube, thus constructing what has since been called tho siphon barometer. As mercury is 10,784 times heavier than air, the height of the atmosphere is 10,784 times 30 inches, or nearly five miles, if it were composed of layers equally dense throughout. But since air becomes less dense as wo ascend, owing to its great elasticity and the diminished pressure, the real height of the atmosphere is very much greater.

From observations of luminous meteors, it has been inferred that the height is at least 120 miles, and that in an extremely attenuated form, it may even considerably exceed 200 miles. Meteors, on entering the earth’s atmosphere, are entirely invisible until they r reach that height of the earth’s atmosphere at which the density of tho air is enough to create considerable resistance. They travel at a velocity of one hundred times that of sound. At this speed the resistance set up creates such a heat as to melt or break up the meteor and at this stago become visible. Thus tho earth’s atmosphere is our protection from falling meteors. A well known authority making observations at Athens many years ago estimated, after 17 years’ observation, that fourteen was the average hourly number visible, and the number coming into the whole earth’s atmosphere was 20,000,000 daily, each of which, under favourable conditions of absence of sunlight, moonlight, clouds, and mists, would furnish a shooting star visible to the naked eye.

Sir John Herschel has calculated that tile total atmospheric pressure exerted on tho average human framein common with all objects on the earth’s surface—is about 14 tons; but, as the pressure is exerted equally in all directions, and permoatox the whole body, no inconvenience arises in consequence of it. The pressure per square inch being about lljflhs.

The aneroid barometer used for our daily readings is made by J. ,J. Hicks, Hatton Garden, London, and is iitted with Mr J). C. Bates’ (N.Z. Gov. meteorologist) patented compensated balance. The dial of the instrument moves quickly for more wind, rises with south winds, and falls with north winds. This class of barometer was invented by Yidi, and patented i* England in 1844. It’s action depends upon the effect produced by the pressure of the atmosphere on a circular metallic chamber partially exhausted of air and hermetically sealed. The surface of the chamber is corrugated in concentric circles. This box, or chamber, being partially exhausted of air, through a short tube, which is made air-tight liy soldering, constitutes a spring, which is affected by every variation of pressure in tlie external (atmosphere, the corrugations or surface increasing its elasticity. At the centre of the upper surface of the exhausted chamber there is a solid cylindrical projectian to the top of which the principal lever is attached. '['his lever rests partly on a spiral spring, and is supported by two vortical pins, with perfect freedom of motion. The end of the lever is attached to a second or small lever, from which a chain extends, where it works on a drum attached to the axis 1 of the hand, connected with a hajr 1

spring, changing the motion from vertical to horizontal, and regulating the hand, the attachment 3of which are made to the metallic plate. The motion 1 rigitmtes in the corrugated elastic box, the surface of which is depressed or elevated as th weight of the atmosphere is increased or diminished, and this motion is com-

municated through the levers to the axis of the hand. The spiral spring on which the lever rests is intended to compensate for the effects of alterations of temperature. The actual movement at the, centre of the exhausted box, from whence the indies lions emanate, is very slight, but by the action of the levers this is multiplied (if>7 times at the point of tlio hand, so that the movement of the 220th part of an inch in the box carries the point of the- hand through three inches of the dial. 'I 110 client

of this combination is to multiply the , smallest degrees of atmospheric pressure, so as to render them sensible on tlie index. I

Owners of this class of instrument will note these barometers require to he compared occasionally with a mercurial barometer, being liable to changes from the elasticity of the brass chamber changing, or from changes in the system of levers which work the pointer. Though aneroid barometers arc constructed showing great accuracy in their indications, yet none can claim to the exactness of mercurial barometers. The mechanism is liable to get fouled and ethor-f wise go out of order, so that they may change 0.300 inch in a few weeks, or oven indicate pressure so inaccurutoly and so irregularly that no confidence can ho placed in them for oven a few days, if the means of comparing them with a mercurial barometer he not at hand.

At the local government buildings there is a standard mercurial barometer, with which owners may compare. and adjust their instruments.

There are two regions of high atmospheric pressure, one north and the other south of the equator, passing comp lately round the globe as broad belts of high pressure. They enclose between them the low pressure of tropical regions, through the centre of which runs a narrower belt of still lower pressure, towards which the north and south trades 1 low. Ihe south belt of high pressure lies nearly parallel to the equator, and is of nearly uniform breadth throughout; but tlio belt north of the equator has a very irregular outline, and great differences in its breadth and in its inclination to the equator—-these irregularities being duo to the. unequal distribution of land and water in the northern hemisphere. Taking a broad view of the subject, there are only three regions of low pressure, one round each pole, bounded by or contained within the belts of high pressure referred to, and the equatorial belt of low pressure. The most remarkable of these, in so far as is yet known, is the region of low pressure surrounding the South Pole, which appears to remain pretty constant during the whole year. Over the ocean, if we except the higher latitudes, atmospheric pressure j„ more regular throughout ihe year than over the land, in the ocean to westwards of each of the continents there occurs at all seasons an area of high pressure .from O.lOin. to 0.30 m. higher than what prevails on the coast westward of which it lies. The maximum is reached during the winter months, and these areas of high pressure are most prominently marked west of those continents which have the greatest breadth in 30 degrees lati-tm-e, and the steepest barometric gradients are on. the eastern sides. It is scarcely possible to over-estimate the importance of t'le.se regions of high and low mean pressures, from their intimate bearing on atmospheric physics, but more particularly from their vital connection with prevailing winds, and the general circulation of the atmosphere. This relation will be apprehended when it is considered that winds are simply the (lowing away of the air from regions where there is a surplus (regions of high pressure) to where there is a deficiency of air (regions of low pressure). Everywhere over the globe this transference takes place. The wind neither blows round the space of lowest pressure in circles returning on themselves, nor does it flow directly towards that space: hut it takes a direction intermediate, approaching, however, more nearly to the direction and course of circular curves than of radii to a centre. More exactly, the angle is not a right angle, but from 45 to 80 degrees. As regards the ocean, the prevailing winds indicate the direction of the drift-currents, and other surface currents.

Frequent readings of the barometer hereabouts goes to show that when, the instrument indicates a low pressure it is followed by a wind, which wind is most times accompanied by rain.

The barometer is a valuable instrument as an indicator of coining weather, provided of course its readings are interpreted with intelligence. High pressures generally attend lino weather, but they not infrequently accompany wet stormy weather; ou the other hand, low pressures, which usually occur with wet and stormy weather, not infrequently accompany line mild weather, particularly in winter. The truth is the barometer merely indicates atmospheric pressure directly whilst it indicates weather only inferentially. The chief points to he attended to are its fluctuations taken in conjunction with the wind and the state of the sky, but above all, tho readings of the barometer as compared with those of neighbouring places, since it is difference of pressure, or the amount of the barometric gradient, which determines the strength of the wind and the weather generally.

A glance at the dial printed daily will show that pressures range over 4 inches, each inch being divided into tenths.

The readings published in this journal are taken daily at mid-day (12 noon), and those possessing instruments may make comparisons of the rise or fall at the hour of the reading of this paper, and thereby become more intimately interested in the question: “What’s the weather going to be? ”