April 10, 1885.] 



• KNOWLEDGE ♦ 



301 



months the moisture-ladeu winds blow lower, and therefore 

 precipitate their vapour earlier; whereas in summer the 

 clouds range higher, and therefore travel farther inland 

 before they fall in rain. The same etlects are observable in 

 the Scandinavian peninsula, Xorway receiving more rain in 

 winter than in summer ; while Sweden, on the eastern side 

 of the Dovrefelds, receives more raiu in summer than in 

 winter. 



Such are some of the general laws which altect the 

 downfall of rain in various countries juid at dill'erent 

 seasons. There is one circumstance involving the action 

 of a yet grander law — about which, however, con- 

 siderable uncertainty still exists. I refer to the difl'er- 

 ence observable between the northern and the southern 

 hemispheres. It has been already noted that the mean 

 position of the medial zone of calms and heavy diurnal 

 rainfalls lies some 1° or 5^ to the north of the equator. 

 The total annual downfall of rain north of this medial line 

 is slightly greater (so far as our present information 

 extends) than the downfall south of the medial line. And, 

 therefore, since the area of the northern region is less than 

 the arei of the southern, it is clear that the annual down- 

 fall over any northern zone is, in general, considerably 

 heavier than the downfall over the corresponding southern 

 zona Now, if we remember that the amount of aqueous 

 vapour raised by evaporation over the southern or watery 

 hemisphere must necessarily be much greater than the 

 amount raised over the northern hemisphere, this result 

 will appear a remarkable one. One would expect to find a 

 dilTerence — and a very marked diti'erence — between the 

 two hemi>pheres ; but instead of the excess of rainfall being 

 in favour of the northern hemisphere, one would expect it 

 to have been in favour of the southern. 



If we assume \«th Maury that the north-easterly and 

 south-easterly trade winds which meet near the equator 

 merge, respectively, into the north-westerly and south- 

 westerly counter-trades — that is, that they cross over to 

 the opposite hemisphere to that in which they were gene- 

 rated — the difficulty seems to vanish. For, in this case, 

 the downfall over the northern hemisphere is due to 

 evaporation over the southern hemisphere, and vice versd. 

 Maury adduced other arguments in favour of his theory of 

 an intercrossing of this sort. Sir John Herschel, however, 

 declined to adopt " the doctrine recently propounded of a 

 systematic crossing of the south-east and north-east trades 

 at the medial line. In so doing," he was " in no wav 

 disturbed by the phenomenon of infusorial dust of South 

 American origin which occasionally falls on the north east 

 of Africa," and so on. I must confess that the balance of 

 evidence seems to me to lie on Maui y's side in this instance. 

 It may be asked, however, whether there is any occasion 

 to adopt either view as a systematic account t>f the laws 

 affecting the trades and counter-trades. May not Maury 

 and Herschel be like the two knights who saw opposite 

 sides of the same shield, and who — both right and both 

 wrong — were persuaded, one that the shield was silvern, 

 the other that it was golden'! 



If we remember that the medial line marks a zone of 

 calm towards which, from either hemisphere, immense 

 mas.ses of moisture-laden air are continually being swept in, 

 why should we arbitrarily assign to the masses of air pass- 

 ing away above from this calm zone, such a law of motion 

 that every particle of air which has originally come from the 

 northern hemisphere shall take one course, and every particle 

 which has come from the southern shall take an opposite 

 one ? It appears to me, on the contrary, that an inter- 

 mingling (in masses, it may be, but still complete) must 

 take place above, and result in an almost indifferent diffu- 

 sion of the vapour-laden air northwards and .southwards 



with the returning counter-trades. The fact that the 

 northern trades have a southernly motion as thoy entci- the 

 calm zone (passing here upwards), and vice veisd, may had 

 to a slight prein>nderance of air (originally) from the 

 northern hemisphere in the north-westerly counter trade, 

 and vice vevsd, but by no means (1 should think) to uny- 

 ihing approaching the systematic intercrossing imagini d by 

 Maury. On the other hand, the preponderance might lie 

 the other way, owing to the effects of collision between the 

 northern and southern trades — but without leading to the 

 systematic return of northern air to the northern tern iterate 

 zone, and of southern air to the southern temperate zone, 

 conceived to take place by Sir J. Herschel. 



One of the most remarkable results of observations 

 made upon rain, has been the discovery that the amount 

 of fall at any place diminishes largely as the rain-gauge is 

 raised above the level of the ground. It is not very easy 

 to ex]>lain this remarkable fact. The explanation oHercd 

 by K;lmtz was, that a falling drop carries with it the tem- 

 perature of the upper regions of air, and condenses on its 

 surface the aqueous vapour present thioughout the lower 

 strata of the atmosphere, as a decanter of cold water docs 

 when brought into a room. Professor Nichol remarks that 

 this explanation " is not an hypothesis but a rigorous 

 deduction, giving an account of all the facts as yet ascer- 

 tained in connection with this subject." But unfortunately, 

 the explanation, though it undoubtedly presents a vera 

 causa, will not bear the test of " quantitative analysis." 

 Sir John Herschel has gone through the simple calculation 

 required to overthrow the theory, and points out thnt, if 

 we allow to the cause the full value it can possibly have (a 

 value far exceeding that which can probab/i/ be attributed 

 to it), we obtain an effect only one-seventeenth part of what 

 is wanted to account for the phenomenon. Sir .John points 

 out also that obliquity of fall cannot possibly affect the 

 observed amount of rainfall, and he offers no hypothesis in 

 exjilanation of the phenomenon, and remarks, in conclusion, 

 that " visible cloud rests on the soil at low altitudes aliove 

 the sea-level but rarely ; and from such clouds alone would it 

 seem possible that so large an accession of rain could arise." 

 He refers, however, in a note to a paper read by Mr. 

 iJaxendell to the Literary and Philosophical Society of 

 ^Manchester on this subject, in which it is inferred that the 

 only way of accounting for the phenomenon lies in the 

 admission of the existence of water " not in the state of 

 true vapour," but already deprived of its latent caloric, 

 though not affecting the transparency of the air, so that " a 

 shallow stratum of the lower and comparatively clear atmo- 

 sphere " may " supply as much rain as a densely-clouded 

 and much deeper stratum in the higher regions." Mr. 

 Baxendell mentions also the interesting fact, that the drops 

 of water which drip from the upper part of the shaft 

 increase to an extraordinary size in the descent to the 

 bottom. 



It appears to me that the well-known phenomenon of 

 rain falling from a clear sky — a rain termed by the French 

 screi'ii — has a suggestive bearing on the peculiarity we have 

 been considering. It proves that water may exist, even in 

 drops, in the atmosphere, without appreciably affecting its 

 transparency. And though it may be an uncommon thing 

 for rain to fall without appearing tirst in the upper regions 



of air in the form of cloud, yet it by no means follows 



that durin/i a shower rain might not be falling from the 

 lower as well as from the upper air-strata, without the 

 transparency of the lower strata being much or at all 

 affected. I have noticed, always, that if the eye be directed 

 steadily at the drops of heavily-falling rain, there will be 

 seen flitting, as it were, among them minute specks, which 

 are seen on a closer observation to be small particles of 



