Aug. 



lo. 1876] 



NATURE 



317 



winds of elastic expansion, such as are the hot winds of 

 India and Australia ; winds which are distinct from con- 

 vection currents, though, it may be, coexisting with and 

 accelerating them. The relations of these winds to the 

 barometric tides are very marked, but it does not seem 

 that the differences of tidal pressure would suffice to 

 generate them, were there not a movement of the air in 

 the same direction arising from more persistent differ- 

 ences of pressure. They probably also depend much on 

 local and irregular differences of pressure. 



The air thus removed in the day-time from continental 

 areas must, of course, collect over the nearest areas of 

 evaporation, with the effect of diminishing the mid-day 

 fall of pressure over those tracts ; and thus seems to be 

 explained those apparent anomalies in the magnitude of 

 the mid-day semi-oscillation of the barometer to which, 

 in the passages quoted from Mr. Buchan's memoir, he 

 has drawn attention, viz., in the case of the Mediterranean 

 area and the Atlantic coast of North America. 



The direction in which this movement of the air takes 

 place will, of course, vary with the locality, but there will 

 always be, on an average, a greater diurnal movement 

 towards cast coasts than towards those facing to the west. 

 This may be illustrated by the case of Calcutta and 

 Bombay, and it is more extensively illustrated by the pre- 

 dominant westerly direction of the land-winds of India, 

 and the cold westerly diurnal winds ^ that blow across 

 the high plains (17,000 to 19,000 feet) of the Changchenmo 

 and Rupshu in Western Tibet. The reason is sufficiently 

 obvious. As the great waves of pressure advance from 

 east to west, the local barometric gradient of any place 

 (in so far as it is determined by the diurnal oscillation) 

 will be expressed by a tangent to the existing phase of 

 the wave. During the hottest part of the day, viz., from 



9 or half-past 9 to half-past 4 or 5, this gradient (which 

 is the steepest and most prolonged of the four) inclines 

 to the eastward, and increases the declivity towards east 

 coasts arising from the excess of pressure over the land. 

 In the opposite direction, viz., towards west coasts, it 

 goes to diminish that declivity. At night the case is 

 reversed. The west to east barometric gradient from 



10 P.M. to half-past 3 or 4 A.M. is in the same direction as 

 that tending to produce an influx of air from the sea 

 towards the land on west coasts; this, however, is 

 opposed to the land wind of the coast line, which is a 

 true convection current, and arises from quite different 

 causes ; and, although traceable in the wind variation at 

 Bombay, it there manifests itself only by decreasing the 

 velocity of the former. There are, moreover, indepen- 

 dent grounds for the inference that this compensating 

 in-flow chiefly affects the higher strata of the atmosphere, 

 while the day wind is chiefly produced in the lower and 

 more heated strata. At Calcutta the easterly (or negative 

 westerly) tendency of the wind at night is very prominently 

 exhibited in the curve of diurnal variation, but although 

 of longer duration it is at no time so intense as the 

 westerly tendency in the early afternoon hours. 



In like manner may be explained the difference of 

 epoch of the corresponding phases of the semi-diurnal 

 east and west variation at Calcutta and Bombay. The 

 gradient of pressure, in so far as it depends on the semi- 

 diurnal oscillation, will, of course, be to the west with a 

 rising pressure, and to the east with a falling pressure, 

 and this normal tidal gradient is affected by the small 

 difference of amplitude over land and sea, in such manner 

 that its changes will be accelerated as affecting east coasts, 

 and retarded as affecting west coasts. Now if we sup- 

 pose that the acceleration in the one case and the retar- 

 dation in the other amount to an hour or an hour and a 

 half, and that the interval between the change in the 

 direction of the gradients, and their effects on the wind, 

 as manifested by the anemometer, is also about an hour 



' This I state on the authority of Dr. Cayley, who assures me that on the 

 high plains these afternoon winds are always from the west. 



and a half, we should roughly reproduce the conditions 

 shown to exist at Calcutta and Bombay respectively. 



According to this view, the local static pressure of the 

 atmosphere, apart from irregular movements, is shown 

 by the height of the barometer at the hours of minimum 

 pressure, and the difference of these expresses the weight 

 of the atmosphere removed and restored by the oscillatory 

 movements chiefly between land and sea. 



I should add, in conclusion, that this communication is 

 merely a risume of some of the more salient topics dis- 

 cussed in two papers, " On the Winds of Calcutta," and 

 " The Theory of Winds of Elastic Expansion," which will 

 shortly be published in extenso elsewhere. 



H. F. Blanford 



CARBONIFEROUS LAND SHELLS 



IN a recent visit to the South Loggius, in Nova Scotia, 

 in which I was assisted in the examination of the 

 cliff by Mr. Albert J. Hill, Manager of the Cumberland 

 Coal Mine, we found a number of well-preserved shells of 

 Ptipa vetusta, in the indurated clay, filling an erect sigil- 

 laria, in a bed considerably higher than those in which 

 the shell was previously known. It is nearly in the middle 

 of group xxvi. of my section of the South Loggius, 222 

 feet above the main coal-seam, 842 feet above the bed in 

 which the species was first recognised by Sir C. Lyell and 

 myself, and about 2,000 feet above the lowest bed in which 

 I have yet found it. It thus appears that this little pul- 

 monate continued to flourish in the carboniferous swamps, 

 after its remote ancestors had been covered with 2,000 

 feet of sediment, including many beds of coal, and nearly 

 the whole thickness of the productive coal-measures. 

 CoJiulus prisats, the only other land-snail found in this 

 section, on the other hand occurs only, so far as known, 

 in the lowest of the beds above-mentioned. Two other 

 carboniferous land-shells, Picpa verviilioneiisis^ Bradley, 

 and Daivsonella Meekt, Bradley, have been found in the 

 coal-field of Illinois ; "and it is worthy of remark that, 

 according to Dr. P. P. Carpenter, all the four species 

 belong to distinct generic or sub-generic forms, and that 

 all these forms are still represented on the American 

 Continent. 



On the same visit, we were so fortunate as to find 

 another large sigillarium stump, rich in reptilian remains, 

 which it is hoped may, on examination, afford new specier> 

 and further information on those already known. 



J. W. Dawson 



THE BIRDS OF KERGUELEN'S LAND 1 



AS regards the publication of results achieved by the 

 naturalists accompanying the recent Transit expe- 

 dition, our American friends appear to be getting the 

 start of us. While we are engaged in issuing " prelimi- 

 nary reports," they have already arranged and classified 

 their collections, and are beginning to publish their dis- 

 coveries. The specimens of birds obtained by Dr. Kid- 

 der, surgeon and naturalist attached to the astronomical 

 party at Kerguelen's Land, or Desolation Island, have 

 been placed for determination in the hands of Dr. E. Coues 

 — one of the most competent zoologists in the United 

 States — and the result has been the very interesting me- 

 moir now before us. We knew already that Kerguelen's 

 Land was not an inviting place of residence for the more 

 highly organised animals, and that few birds were to be 

 found there. We know now what those few are, and 

 have full particulars about most of them, their lives, and 

 habits. According to Dr. Coues' determination, Dr. 



' " Bulletin of the United States National Museum," No. 2. Contribu- 

 tions ta the Natural History of Kerguelen Island, made in connection with 

 the American Transit of Venus Expedition, 1874-75. By J. H. Kidder, 

 M.D. I. Ornithology. Edited by Dr. Elliott Coues, U.S.A., 8vo. 52 pp. 

 (Washington, 1875.) 



