HI'S 



METEOROLOGY. 



[THK ATMOSPHERE. 



tiering that the atmosphere, in accordance with tin- 

 law* of elasticity, must necessarily expand, the higher 

 we ascend above the normal level of the surface of uur 

 globe or, in other words, tin- l.-\. 1 of tho sea the first 

 question which arises is this To what extent do the at- 

 mospheric limits reach ? does the expansion go on, ad 

 tnrfm/iim, to the farthest realms of space, or are these 

 limits df finite ? and if definite, what is the cause, or 

 what are the causes, of limitation i Two theories hare 

 been adopted in reference to this question. According 

 to one theory, the atmosphere is illimitable ; according 

 to the other, it is limited. Some of the arguments for 

 and against we shall now proceed to give.* 



If the atmosphere be really illimitable, let us see what 

 should follow, to be in accordance with recognised laws, 

 to which all ponderable matter, or matter subject to 

 gravitating influence*, is amenable. Gravitation being 

 directly as the mass of gravitating bodies, it should 

 follow that, were the atmosphere illimitable, each of the 

 heavenly bodies should be surrounded with an atmo- 

 sphere proportionate to its mass an assumption which 

 astronomy disproves. Thus, astronomy furnishes strong 

 proof in favour of the finite extension of the atmosphere. 

 A consideration of the laws of the atomic constitution of 

 matter, lends further, and peihaps stronger, proofs. 

 Chemistry is full of evidence in favour of the atomic con- 

 Ktitution of matter ; or, in other words, is full of proofs 

 that all material substances are composed of molecules or 

 particles ; to which extent they can alone be divided, and 

 not beyond. The mathematical reasoning, which has 

 been employed against this atomic theory, as chemists 

 term it, is specious at a first glance, but really untenable. 

 To argue that the theoretical space, occupied by any mate- 

 rial particle, may be supposed capable of division and sub- 

 -n, is really not to the point. Space 

 is nc thing ; the matter occupying such space is another. 

 The mathematical objection touches the space alone, not 

 the matter ; therefore the chemical evidence in favour 

 of the atomic constitution of matter is unanswered, and 

 is apparently unanswerable. Let us now regard the con- 

 sequences of this assumption as it relates to atmospheric 

 uir. The late Dr. Wollaston was the first person who 

 directed attention to the limitation which should, theo- 

 retically, be imposed on atmospheric expansion, supposing 

 the assumption of its atomic constitution to be correct. 

 The atmosphere, like other ponderable material bodies, 

 is subject to gravitating influences, thus imparting a 

 tendency of descent towards the earth. On the other 

 hand, the atmosphere being elastic, its particles are 

 mutually separated from each other by the operation of 

 this force. Now, assuming the atomic constitution of 

 the atmosphere to hold good, there must be Rome finite 

 distance from the earth's surface, at which the force of 

 elasticity would be counterbalanced by the force of 

 gravitation, which distance would correspond with the 

 fart h- t limits of the atmosphere. The mean distance 

 of thin limit is assumed to be about forty-five miles, 

 though it must ditler from every point north and south ; 

 being greatest over the equator, and least at either pole, 

 as indicated by the accompanying diagram (Fig. ID). 



The reason of its being greatest at the equator is im- 

 mediately referable to the diurnal rotation of our globe 

 on its axis, thus generating a centrifugal force, which 

 lias determined ihe oblate spheroidal form of the earth. 

 Material bodies will be affected by this centrifugal force, 

 cuttrii parihtis, directly as their attenuation ; whence it 

 follows that the atmosphere, being a gas, must be affected 

 to an extreme degree. It will be sufficiently evident, 

 however, that the atmosphere is only alieetud |,y the 

 earth's diurnal rotation intermediately, or by friction ; the 



'y of motion imparted to it by the earth being less I 

 considerable than the velocity of the earth itself. \\ V 

 ball hereafter find, when we come to treat of the trade- , 

 winds, that those ]>erinanent aerial currents are not alto- 

 i- referable to atmospheric motion, but in sonic 

 demo depend upon the ilnirnal motion of the earth. 

 Vtttrm\nalu>n of th* IIVi<//"< of a yi M of , 



Air. Nothing can be more easy than the 



He* ; Htum^licl, mull, i 



theoretical means of solving this prohVm ; and. though 

 certain practical difliculties do miei-. . we had I, 



!. 



for the sake of theoretical explanation, consider them 

 absent 



The case under consideration is general, not specific. 

 The determination of the weight of a given volume of 

 atmospheric air, is accomplished similarly to thedotermi- 

 nation of the weight of a given volume of any other gas ; 

 nor does it differ in principle from the process had re- 

 course to when solids or fluids are concerned. 



If, to take the simplest practical case, without reference 

 to cohesive state, it were desired to ascertain the weight 

 of a given bulk of copper or brass say one hundred cubic 

 inches the operator's first care would be to obtain a 

 solid of copper or brass having these cubic dimensions ; 

 which having been obtained, no vessel for the purpose of 

 weighing it in would be necessary. This is the simplest 

 case of bulk-weighing which can occur ; nevertheless, a 

 conditiou has to be regarded which tho superficial 

 observer might forget, or, perhaps, not be aware of. The 

 copper or brass alters its dimensions for every variation 

 of temperature. If heated, it will expand ; if cooled, it 

 will contract ; so that, practically, under no two degrees 

 of temperature has the mass of brass or copper the same 

 size. Practically, so long as solids are concerned, these 

 variations of size, dependent upon variations of teun 

 ture, are not of much consequence in ordinary operations 

 of weighing. It is necessary, however, to estimate them 

 for other reasons, and to tabulate these variations. \Yo 

 shall have occasion to refer to this tabulation here 

 after. 



Let it now be assumed that the problem before us is to 

 determine tho weight of a given bulk of liquid say 

 water. In this case we must have recourse to s 

 vessel of capacity, for the purpose of holding the water 

 to be weighed ; and further elements of complexity, in 

 addition to that of temperature, are introduced. Firstly, 

 the vessel employed has its own laws of expansion and 

 contraction ; secondly, the water, when poured into the 

 vessel, will not have a perfectly flat surface ; so that, 

 except tho mouth of the vessel be small, an error of 

 considerable magnitude will be imparted ; neither must 

 it be too small, or the functions of capillary attraction 

 will como into play, and then the water will present a 

 higher level than properly belongs to it. 



The chief source of inaccuracy, however, which the 

 operator meets with in operating upon solids and liquids, 

 is that dependent on the variations in bulk referable to 

 thermal increments and lieerements ; any alteration due 

 to variations in pressure being practically ignored. Ni 

 far as atmospheric pressure is concerned which is tho 

 only kind of pressure we need take cognizance of as 

 affecting our subject it exercises so little influence on the 

 dimensions of solids ami liquids, that we may put it alto- 

 r out of consideration. Far ditleivnt, however, is it 

 wii'-n gases are concerned. Their attenuation and elasti- 

 city are such, that variations of atmospheric pressure 

 exercise the most powerful influence over thorn ; so that 



