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prevail, however, the alternation of dry and wet seasons depends on the winds. When the south-west monsoon is bioving over India, for instance, there is no rain on the east coast, but abundant rain on the west coast. During the north-east monsoon these conditions are reversed. A little consideration will show that all the above-mentioned seasonal variations within the tropics depend on general laws already stated.
Beyond the tropics there is less regularity. The fall of rain depends on the prevalence of certain winds which bring moisture with them, and these winds not blowing with any regularity, the rainfall is similarly irregular. In countries close to the tropics, there is a noteworthy dryness in summer; for this reason clearly, that in summer the trades blow over these regions, and bring with them “ trade-wind weather.” Further north, however, though there may be a tendency to the prevalence of north-easterly winds in summer, this tendency is not so marked as to produce a considerable defect of rain in the summer as compared with the winter months.*
In England we have one cause affecting the rainfall which is worthy of special notice. I refer to the Gulf-stream. The air above this warm stream is not only warmer than the surrounding air, but is heavily laden with moisture. When the western and south-western winds loaded with the vapour of water begin to blow over England, they precipitate their moisture in rain as they encounter the colder air over the land; but the manner in which this happens is variable with the seasons, for in the winter months the moisture-laden 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 effects are observable in the Scandinavian peninsula, Norway receiving more rain in winter than in summer; while Sweden, on the eastern side of the Dovrefields, receives more rain in summer than in winter.
Such are some of the general laws which affect the down. fall of rain in various countries and at different seasons. There is one circumstance involving the action of a yet grander law -about which, however, considerable uncertainty still exists. I refer to the difference 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 4° or go to the north of the equator. The total annual downfall of rain north of this medial line is
* So far as my own observations extend, I should say that the two features of our climate which may be most certainly depended on- which, be it noted, is not saying much-are, heavy rains in July, generally in the last fortnight, and serene weather during the second week of November.
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 area of the southern, it is clear that the annual downfall over any northern zone is, in general, considerably heavier than the downfall over the corresponding southern zone. Now, if we remember that the amount of aqueous vapour raised by evapo. ration 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 difference--and a very marked difference-between the two hemispheres; 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 with Maury that the north-easterly and southeasterly 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 generated, 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 versa. Maury adduces other arguments in favour of his theory of an intercrossing of this sort. Sir John Herschel, however, will not listen to Maury's views. He“ declines adopting the doctrine recently propounded of a systematic crossing of the south-east and north-east trades at the medial line. In so doing,” he is “in no way 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 mo to lie on Maury's side in this instance.
It may be asked, however, whether there is any occasion to adopt either view as a systematic account of 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 masses of moisture-laden air are continually being swept in, why should we arbitrarily assign to the masses of air passing 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
thall completeillusion of turning conly motion versa, may
the contrary, that an intermingling (in masses, it may be, but still complete), must take place above, and result in an almost indifferent diffusion of the vapour-laden air northwards and southwards with the returning counter-trades. The fact that the northern trades have a southerly motion as they enter the calm zone (passing here upwards), and vice versa, may lead to & slight preponderance of air (originally) from the northern hemisphere in the north-westerly counter-trade, and vice versa, but by no means (I should think), to anything approaching the systematic intercrossing imagined 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 temperate 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 abore the level of the ground. It is not very easy to explain this remarkable fact. The explanation offered by Kämtz is, that a falling drop carries with it the temperature of the upper regions of air, and condenses on its surface the aqueous vapour present throughout the lower strata of the atmosphere, as a decanter of cold water does when brought into a room. And of this explanation Professor Nichol remarks, that “it is not an hypothesis but a rigorous deduction, giving an account of all the facts as yet ascertained 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, that if we allow to the cause the full value it can possibly have (a value far exceeding that which can probably be attributed to it) we obtain an effect only oneseventeenth 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 explanation of the phenomenon, and remarks in conclusion, that “visible cloud rests on the soil at low altitudes above 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. Baxendeil to the Literary and Philosophical Society of Mancbester 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
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of its latent caloric, though not affecting the transparency of the air, so that “a shallow stratum of the lower and comparatively clear atmosphere ” may “supply as much rain as a densely-clouded and much deeper stratum in the higher regions.” 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 serein, 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 first in the upper regions of air-in the form of cloud, yet it by no means follows that during 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 water. Now, it does not appear to me likely that these, or most of them, are produced by the collision of the falling drops—for the paths of two neighbouring drops must be parallel, since the drops are subjected to precisely the same set of influences.
I believe the phenomenon to be one worthy of more careful notice than it has received-in fact, I am not aware that it has been noticed at all. The motions of the particles are them. selves interesting-seeming almost as independent of gravitation, wind-currents, or the like, as the motion of a flight of insects would be. It is hardly necessary to observe that if these particles show that rain is being generated in the lower as well as the upper strata of the air, all difficulty in explaining the results of Professor Phillips's observations, vanishes at once.
TIIE GRAVE-MOUNDS OF DERBYSHIRE, AND
BY LLEWELLYNN JEWITT, F.S.A., ETC., ETC.
(Continued from page 266.)
THE ROMANO-BRITISH PERIOD.
hire belong Caxon, and by 1.
As I have said earlier on, the greater part of the Grave-mounds of Derbyshire belong to the Celtic period; the intermediate number to the Anglo-Saxon, and by far the least of all to that now under notice, the Romano-British. There is, both in the lowlands of the county, and in the higher or mountainous districts of the Peak, abundant evidence of Roman occupation, and of the arts practised by that people, but very little know. ledge indeed is to be gained there as to their funeral customs or their modes of sepulture. Of the living Roman and of his avocations, indications are not unfrequently brought to light by the burrowing miner, or by the surface-working agriculturist, but of the dead it is rarely indeed that any remains are exhumed.
In Derbyshire the Roman was, it would seem, more of a “bird of passage” (as well as, to some extent, a “ bird of prey'') than a settler, and the consequence is that no remains
or next to no remains-of villas or of settlements are found, and that where burial has taken place it has not unusually been in the same mound with those of an earlier period. The Ancient Briton raised the mounds over the remains of his own people, and his Roman subjugator, as occasion required, took possession of them, and therein laid his own dead. Thus the same barrow is sometimes found to contain, besides its primary Celtic interment, and others belonging to the same race, later deposits (nearer to the surface or to the side) of the RomanoBritish or of the Anglo-Saxon periods.
The Roman roads of Derbyshire were many in number, and some of them are of considerable importance. The principal line (as well, most probably, as some of the others) was formed on an old British way; while other roads were constructed by them for the convenience of working, and for greater facility in transporting the produce of, the mines, in which a profitable trade was carried on. The principal road, the Rykneld Street, entered the county from Staffordshire, and