y^an. 1 1, 1872J 



NATURE 



201 



cipitation than any he has instanced takes place every 

 summer, and does probably cause a very great depression 

 of the barometer, but certainly does not give rise to any 

 winds such as he has described. On the hills of Khasia, 

 again, where the unparalleled rainfall is as much as from 

 30 to 40 inches a day for days together, and puts the 

 paltry h or 5 of an inch a day of Mr. Ley's examples 

 almost beyond the pale of comparison, no such storms are 

 generated. In the same way, the e.xplanation of the 

 eastward direction which these barometric depressions 

 take in our latitudes, which differs only in its greater 

 detail from that given by Prof. Mohn in the " Storm 

 Atlas," is applicable only to temperate latitudes ; the 

 westward advance of tropical cyclones cannot be referred 

 to it ; and it seems to us improbable in the extreme that 

 the course of a storm is regulated by one law in one part 

 of the world, and by a totally distinct law in another. Be- 

 sides this, in the detailed application of the law which he 

 deduces for Western Europe, the author appears to fall 

 into the mistake of attributing the rainfall of mountain 

 districts to the mere contact of the moist air with the cold 

 mountain slope ; that this is not the case — that it is due 

 rather to the hoist into the upper regions which the air 

 receives on impinging against the slope — is curiously 

 shown by the fact that, when the hills are not high, most 

 rain falls on the lee side. One familiar instance of this 

 will illustrate our meaning. The gauge which in all Eng- 

 land shows the greatest rainfall is at .Stockley Bridge, 

 just above Seathwaite ; it is distinctly under the lee of the 

 ridge which joins Great Gable to Great End, and separates 

 Wastdale from Borrowdale. The mist, blown in from 

 seaward, fills Wastdale, and is lifted up the slope of this 

 ridge (Stye Head Pass). Crossing over out of Wastdale, 

 the mist curling up the hill is frequently so thick that the 

 path cannot be seen 10 feet in advance ; but immediately 

 on reaching Stye Head Tarn the mist vanishes, to fall as 

 rain over Seathwaite. But altogether, though we admit 

 neither the author's premises nor his conclusions, his 

 work is none the less highly interesting. It does not con- 

 tain much that is new, but it discusses and illustrates the 

 theories of Mohn and Buchan in greater detail than has 

 yet been attempted. We would, however, decidedly object 

 to the ex axtliedra tone which is occasionally adopted. In 

 empirical science very little is " obvious," and perhaps 

 nothing is a " truism ; " certainly the influence attributed 

 to the earth's rotation is neither one nor the other, for it 

 is denied, disputed, and doubted by very many capable 

 meteorologists. J. K. L. 



The Young Collector's Handy-book of Botany. By the 

 Rev. H. P. Dunster. (London : L. Reeve and Co., 1871.) 

 We opened this little book with pleasure, hoping to find 

 in it an addition to the too few popular manuals of botany, 

 and the pleasure was increased by recognising at the end 

 some familiar and excellent illustrations. Great there- 

 fore was our disappointment when we found that instead 

 of "assisting the student in the beginning of his work by 

 setting him forward on a right road," as is stated in the 

 Preface to be its object, it would be far more likely to 

 mislead him. Botany seems to be peculiarly unfortunate, 

 in that every one who is fond of flowers thinks himself 

 capable of writing a handbook, without himself possess- 

 ing any accurate scientific knowledge of his subject. 

 Some of the definitions given in this book are so bad that 

 we should have been surprised to find them in the answers 

 to the examination papers of the botanical classes in any 

 of the great schools where natural science is now taught. 

 Take four examples : — "Albumen : a gummy substance 

 surrounding certain seeds;" "Embryo: the leaf in an 

 immature state ; " " Matrix : that upon which any other 

 thing grows ; " " Petals : leaves while in the corolla." After 

 this we are somewhat prepared to hear that the corolla 

 " is made up of petals which, when expanded, are the 

 flower-leaves, and of the stamen and pistils ; " and that 

 "county collections (of ferns) are valuable as illu;trations 



of the fauna of particular parts." We are utterly un- 

 able to see the object gained by the publication of this 

 book, when beginners already have such admirable 

 manuals as Oliver's '■ Lessons in Elementary Botany," 

 Lindley's " School Botany," and Cooke's '' Manual of 

 Structural Botany," neither of which, by the way, is 

 mentioned by Mr. Dunster in the list of books recom- 

 mended to the learner. Especially are we unable to 

 understand how the names of respectable publishers, 

 who have issued many admirable works on natural his- 

 tory, come to be appended to a book of this character. 

 As we see that it is intended to be the first of a series of 

 Handy-books upon " the popular and recreative sciences," 

 we would recommend the publishers to submit the manu- 

 script of the remainder of the series to a competent 

 judge before publication. A. W. B. 



LETTERS TO THE EDITOR 



[ The Editor docs not hold himself responsible for opinions expressea 

 by his coi-respondciits. No notice is taken of anonymous 

 commnnicaiions. ] 



Ocean Currents 



Leaving out of account a few small inland seas, the globe 

 may be said to have but one sea, as well as but one atmosphere. 

 We have, however, accustomed ourselves to speak of parts, or 

 geographical divisions, of the one great ocean, such as the At- 

 lantic and the Pacific, as if they were so many separate oceans. 

 We hive become accustomed, also, to regard the currents of the 

 ocean as separate, and independent of one another ; and this 

 idea has, no doubt, to a considerable extent, militated against 

 the acceptance of the theory, that the currents are caused by the 

 winds, and not by difierence of specific gravity, for it leads to the 

 conclusion that cun-ents in a sea must flow in the direction of 

 the prevailing winds blowing over that sea. 



The true way of viewing the matter, as I hope to be able to 

 show in my next letter on the cause of Ocean CuiTents, is to re- 

 gard the various currents merely as members of one grand system 

 of circulation, produced, not by the trade winds alone, as some 

 suppose, but by the combined action of all the prevailing wind^ 

 of the globe, regarded also as one system of circulation. 



If the winds be the impelling cause of currents, the direction of 

 the currents will depend upon two circumstances, viz. (i) the 

 direction of the prevailing winds of the globe; and (2) the con- 

 formation of sea and land. It follows, therefore, that as a cur- 

 rent in any given sea is but a member of a general system of 

 circulation, its direction is determined, not alone by the prevailing 

 winds blowing over the sea in question, but by the general 

 system of prevailing winds. It may, consequently, sometimes 

 happen that the general system of winds may jjroduce a current 

 directly opposite to the prevailing wind blowing over the 

 current. 



Taking into account the effects resulting from the conformation 

 of sea and land, the system of ocean currents is f jund to agree 

 exactly with the system of the winds. I trust to be able to show 

 that all the principal currents of the globe, the Gibraltar current 

 not excepted, are moving in the exact direction in which they 

 ought to move — assuming the winds to be the sole impelling 

 cause. Given the system of winds and the conformation of sea 

 and land, the direction of all the currents of the ocean, or more 

 properly the system of oceanic circulation, can be determined 

 a priori. Or given the system of the ocean currents, together 

 with the conformation of sea and land, and the direction of the 

 prevailing winds can abo be determined rt/;v'y;7. Or, thirdly, 

 given the system of winds and the system of currents, and the 

 conformation of sea and land may be, at least, roughly deter- 

 mined. For example, it can be shown by this means that the 

 Antarctic regions are probably occupied by a continent, and not 

 by a number of separate islands, nor by a sea. 



The influence of the rotation of the earth on ocean currents has 

 certa'nly been greatly over-estimated. Rotation, as is well 

 known, exercises no influence in generating motion in any body 

 placed on the earth's surface ; but if this body be already in 

 niodon, no matter in what direction the motion may be, rotation 

 will deflect it to tlie rij;ht on the northern hemisphere, and 

 to the left on the southern hemisphere, as has been shown 

 by Mr. Fen-el. But it must be borne in mind that the 

 deflecting power of rotation depends wholly on the rate a' 



