I'NKI'MATICS. 



the other end becomes worm. The. particles do not change their 

 to one another, bat the heat passes from 

 each i. t<> it, and thua is conducted along tho bar. 



.M'l pisea, however, beconu- l:< a< <l in a totally 

 1 of tho heat bnim; conveyed from .M- 

 to another by contact, onrrouta are aet up in them by 

 ii-li nil tin' | :i rtiolea successively become exposed to the source 

 These currents may easily be aeon by dropping Eome 

 nta of litmua into a globe of water, and heating it by 

 . .f a spirit-lamp placed under the centre. Tho litmuH 

 will colour the water, and thua it will be Boon that there in an 

 . nip column of liquid in the centre of the globe. When 

 the surface it spreads out in all directions, and 

 !M against the sides, to be raised to a higher temperature 

 and again ascend. We may easily see how bad a conductor of 

 <i'iitl in. If wo place a lump of ice at tho bottom of a 

 ii 10 nearly filled with water, and apply tho flame of a 

 spirit-lamp to the upper portion, wo shall find that the ice will 

 rrm iin unmelted long after the water at the top has commenced 

 to boil. If, however, we apply tho flame to tho bottom of the 

 lie ico will very quickly melt. In a similar way it might 

 be shown that air is a bad conductor, and can therefore only be 

 warmed by convection. 



Another important fact in connection with the temperature of 

 the air is that it is not warmed by the passage of the sun's rays 

 through it. If it were, the upper part of tho atmosphere would 

 in tho daytime be warmer than the lower portion, for it is nearer 

 the sun ; we find, however, that as we ascend the temperature 

 becomes less and less. An interesting illustration of this is 

 :i the higher tropical mountains. If we go to some of the 

 lofty Andes near the equator, wo shall find at their base tropical 

 forests almost too dense for the light of the sun to penetrate ; 

 on ascending higher we meet with vegetation less rank, and pass 

 through regions resembling those of the temperate zone. Higher 

 still we come across a stunted polar vegetation, and reach the 

 limit of perpetual frost, or the " snow-line." We have, in fact, 

 in a small space a representation of all the climates we meet in 

 travelling from the equator to the poles. The reason of this is 

 that the rays of the sun pass through the air without warming 

 it, !iut when they reach the ground they are absorbed and raise 

 it--* temperature. This increase is shared by tho air resting on 

 it, and thus tho lower strata become warmed. 

 As we recede from the equator, the snow-line, 

 which there attains an elevation of 17,000 or 

 18,000 feet, descends lower and lower, till in 

 the frigid zone it comes to the level of the sea, 

 and we find continual frost and winter. 



It remains for us now to see how the princi- 

 ples we have examined explain the complicated 

 phenomena of the winds. 



Wind is merely air in motion ; but this motion 

 varies from time to time very greatly, both in 

 direction and in intensity. There are various 

 modes of measuring tho speed of the wind. A 

 rough idea of it may sometimes be formed by 

 noticing the shadows of clouds passing along 

 the ground ; but this is vague and uncertain. 

 Various machines, known as anemometers, have 

 therefore been contrived for this purpose, and 

 are in use in most observatories. As the pressure 

 produced on any surface varies as the square of 

 the velocity, some of the simplest anemometers 

 merely show the pressure, and from this the velocity must be 

 calculated by tables drawn up for the purpose. Fig. 21 repre- 

 sents one of the most common forms ol this instrument, which 

 is known as Lind's anemometer. It consists of a glass tube 

 bent into the shape of the letter U, but drawn out at the bend 

 so as to diminish the bore. The object of this is to moderate 

 the fluctuations which are caused by the gusts of wind. One 

 end is also turned at right angles, and the whole is suspended 

 on a pivot, so that this open end is always presented to tho 

 wind. A small shield, placed over the end of the other limb, 

 shelters it a little from the wind, and it is found that as soon 

 as the instrument is exposed to the wind, its pressure acts on 

 the liquid and depresses it in the limb exposed to its force. The 

 difference in the level of the water in the two limbs indicates 

 the pressure. If this difference be one inch, the pressure is 

 about 15 Ibs. per square foot. 



Fig. 21. 



A metal disc, supported by a apiral apriof . is likewise used 



-i to meaauro the pressure. Fana arranged like the 



ails of a mill, and giving motion to a registering apparatus, 



have also been tried ; but these forma have, for the moat part, 



given way to that known aa Robinson'* anemometer. 



He found that the vertical vanes moved much more alowly 

 tlnui the wind, and therefore substituted for them metal oops or 

 hemispheres. Four of these, with their open aides turned the 

 same way, are accordingly mounted at the extremities of two 

 arms crossing at right angles, and the axle to which these are 

 fixed is connected with a train of wheels so as to register the 

 revolutions. 



When this instrument is placed in an exposed place, the wind 

 acts on the concave aides presented to it, and thus sets the 

 whole in rotation, the speed of the cupa being nearly equal to 

 that of the wind, and always bearing the same proportion to it. 



This instrument is often fitted up by the aide of a vane, and 

 both are made self-registering. The construction of the appa- 

 ratus for this is very ingenious, but its description would be too 

 long for insertion here. 



The following table gives an idea of the effects produced by 

 different velocities of the wind : 



CtwrscterUtlri. 

 Hardly perceptible. 

 Just perceptible. 

 Oentle breeze. 

 Brisk wind. 



> Very brisk wind. 



High wind. 

 Very high wind. 

 Storm. 

 Great storm 

 . Hurricane. 



Great hurricane, carrying 

 trees, etc., before it. 



Mili-d IK.T 



1 



2 



5 



10 

 20 

 25 

 35 

 45 

 50 

 60 

 80 

 100 



I'n s-siire per Kjumre 

 font in pound*. 



0-005 



0-020 



0-123 



0-492 



1-968 



3-075 



6-027 



9-963 

 12-300 

 17715 

 31-490 

 49-200 



Winds vary in direction as much as in velocity. In temperate 

 climates they are often proverbially uncertain, but in the tropics 

 they are very much more regular ; and we will now notice a few 

 of them, which are distinguished by special names on account 

 of their importance. 



The most important are the trade-winds. These are perma- 

 nent, and extend a few degrees north and south of the tropics, 

 the actual limits varying with the season. Their direction in 

 the northern hemisphere is from tho north-east, and in the 

 southern from the south-east. Hence they are called the north- 

 east and south-east trades. Between them there exists a narrow 

 region of calms sometimes called the Doldrums. This band is 

 a little north of the equator, and nearly corresponds with the 

 line of greatest heat on the earth. 



Columbus was the first who noticed these winds ; and when 

 the sailors found that a constant wind kept on day after day, 

 taking them further from their home, they almost mutinied. 

 So regular are these winds that a ship will often sail from the 

 neighbourhood of the Canaries right across to the north coast of 

 South America without altering her sails. A little consideration 

 will render the origin of these winds perfectly clear. In tropical 

 regions the surface of the earth, being exposed to the almost 

 vertical rays of the sun, becomes intensely heated, and commu- 

 nicates this high temperature to the air around. This accordingly 

 expands and rises, its place being supplied by colder air, which 

 rushes from temperate and polar regions, Now we should at 

 first expect that this would cause the wind to blow from both 

 poles towards the equator that is, a north wind in the northern 

 hemisphere and a south wind in the southern (for a wind is 

 named after the point from which it blows) ; and if the earth 

 were at rest, this would be the case. It is not at rest, however, but 

 rotating rapidly from west to east ; and it is clear that since the 

 diameter of the earth is greatest at the equator, and diminishes 

 as we recede from it, the velocity too must be greatest there. 



We find that any place at the equator has a velocity of nearly 

 1,000 miles per hour, while at 45, north or south latitude, it is 

 only 660. The air, therefore, as it flows towards the equator, 

 has at every point a less velocity than that of the portion of 

 the earth's surface it is moving over, and hence it is left behind, 

 and appears to be moving in a contrary direction to the earth. 

 Now the motion of the earth is from west to east ; the wind, 

 therefore, appears to travel from east to west, and this, com- 



