314 



NATURE 



\Aug. lo, 1876 



slits in the large sharks are small, here they are of 

 immense size. Their function is to allow of sufficient 

 water to flow in and over the gills to oxygenate the fish's 

 blood ; but in Selache they serve also as supports to the 

 strainers ; and as so big a body must require a great lot of 

 food, the in-takings and out-puttings must be many, and 

 might account for the gradual increase in the size of these 

 slits until they reached their present immense proportions, 

 where they have to subserve both the functions of nutri- 

 tion and circulation. The convexity of the gill-openings 

 is towards the shark's mouth, the concavity of these 

 fringed rays is in the same direction. The edge repre- 

 sented in the drawing as jagged — an appearance assumed 

 in drying— is attached to the inner edge of the flaps 

 covering the gill-openings, being somewhat more firmly 

 attached towards the central portion, which in the draw- 

 ing is far too cartilaginous-looking. The gills are outside 

 the whalebone fringes. There seems little reason to 

 doubt but that the free points of the fringes of the one 

 row can be so erected from its gill ray edge as to bend 

 forwards and join, and perhaps slightly interlace with 

 those of the opposite row, and thus there would be a 

 series of arches of whalebone protruding into the neck 

 cavity of the fish. When these fringes are applied to the sur- 

 face of the gill rays, the water could flow without resistance. 

 The gills were quite free from parasites, in this respect 

 differing from the gills of the Rhinodon of the Seychelles. 

 Although this is not the place to enter into minute details, 

 there is little doubt that Dr. Fleming is wrong in stating 

 that the skin seems smooth when the hand is passed 

 from the head to the tail ; and yet though the scales are, 

 as described by Dr. J. E. Gray, armed with small curved 

 points bent in all directions, so that the skin feels rough 

 each way, the hand can be rubbed several times more 

 eisily from head to tail than frorri tail to head, indicating 

 that a larger number of the curved points are directed 

 towards the tail. 



The oil from the liver of a medium-sized Basking Shark 

 is worth nearly 40/. sterling ; but the difficulties and danger 

 of capturing these sharks seem altogether to be greater 

 than those attending the whale-fishery. The same was 

 true at the Seychelles. Men engaged at the sperm-whale 

 fishery off St. Denis often told me they dreaded to harpoon 

 by mistake a Rhinodon. A whale must come up to breathe 

 or else choke itself. But there were stories told me of how 

 a harpooned Rhinodon, having by a lightning-like dive 

 exhausted the supply of rope, which had been accidentally 

 fastened to the boat, dived deeper still, and so pulled 

 pirogue and crew to the bottom — there, in spite of the 

 harpoon in its neck and its attendant incumbrances, it 

 was at home for a great length of time. 



Ed. Perceval Wright 



ON THE PHYSICAL EXPLANATION OF THE 

 INEQUALITY OF THE TWO SEMI-DIUR- 

 NAL OSCILLATIONS OF BAROMETRIC 

 PRESSURE "■ 



THERE are, perhaps, few phenomena in the domain 

 of terrestrial physics which have received more 

 attention than the diurnal variation of barometric pres- 

 sure, and on the causes and explanation of which, never- 

 theless, there is more diversity of opinion even at the 

 present day. Dove, Sabine, Herschel, Espy, Lamont, 

 Kreil, Broun, and many others have in turn engaged in 

 the discussion of this vexed problem, and at the present 

 time Mr. Alexander Buchan is publishing an elaborate 

 and most valuable resum'e of the existing data in the 

 Transactions of the Royal Society of Edinburgh as a 

 preliminary to a renewed investigation. 



The general features of the diurnal variation of pressure 

 are familiar enough to every one who has ever observed 



the rise and fall of the barometer for a few days in India, 

 and most other tropical countries. From about 3 or 4 in 

 the morning the pressure increases gradually towards 

 sunrise, then more rapidly, and culminates generally 

 between 9 and 10 a.m. A fall then sets in, which becomes 

 rapid during the hottest hours of the day, and the pressure 

 reaches its minimum generally between 4 and 5 p.m. The 

 pressure then increases till about 10 p.m., but in general 

 does not attain the same height as at the corresponding 

 morning hour. Lastly, a second fall brings it to a second 

 minimum between 3 and 4 a.m., which, except on moun- 

 tain peaks and at such stations as Simla and Darjiling, 

 is, as far as my own experience goes, never so low as the 

 afternoon minimum.^ 



Thus, then, the pressure rises and falls twice in the 

 tvfenty-four hours, attaining, in general, its absolute 

 maximum about 9 or 9.30 A.M., and its absolute minimum 

 between 4 and 5 p.m. 



This may be taken as a general description of the 

 phenomena as exhibited in the tropics ; but it presents 

 many striking variations at different places, and at one 

 and the same place at different times of the year. These 

 variations affect — the hour at which the pressure attains 

 its maximum and minimum values, the absolute ampli- 

 tude of the oscillations, and lastly, their relative ampli- 

 tude. It is this phenomenon — the variation in the relative 

 amplitude of the day and night oscillations — the probable 

 physical explanation of which I have now to bring to 

 notice. 



It was observed by Arago, apparently some years prior 

 to 1 841, that in Europe " the proximity of the sea has the 

 effect of diminishing the amplitude of the interval during 

 which the diurnal fall lasts, viz., that which occurs between 

 9 a.m. and 3 P.M. ;" and considering the whole pheno- 

 menon as made up of a single and double oscillation, it 

 may easily be shown that this interval is determined 

 mainly by the relative amplitude of these two elements. 

 The latest notice on the subject is given in the following 

 extract from Mr. Buchan's Memoir, a copy of the first 

 part of which (for which I am indebted to the author) 

 has reached me only within the last week. In summing 

 up the characteristics of the midday fall of pressure, he 

 says : — " Whatever be the cause or causes on which the 

 diurnal oscillations of the barometer depend, the influence 

 of the relative distribution of land and water in deter- 

 mining the absolute amount of the oscillation in particular 

 localities, as well as over extended regions, is very 

 great. From the facts detailed above, it will be seen that 

 this influence gives a strong local colouring to the results, 

 particularly along the coasts, and that the same influence 

 is extensively feJt over the Channel, the Mediterranean, 

 the Atlantic, and other sheets of water on the one hand, 

 and on the other over the inland portions of Great 

 Britain, Europe, and the other continents ; " and farther 

 on he adds : " While, as has been pointed out, numerous 

 illustrations can be adduced showing a larger oscillation 

 over the same region with a high temperature and 

 a dry atmosphere than with a low temperature and a 

 moist atmosphere, the small summer oscillation on the 

 coasts of the Mediterranean and those of the Atlantic 

 adjoining is in direct opposition to the idea that any such 

 conclusion is general. For over those parts of the Medi- 

 terranean and Atlantic the temperature is hottest in 

 summer and the air is driest — so dry, indeed, that no rain, 

 or next to none, falls ; and yet there the amplitude of the 

 oscillation now contracts to its annual minimum. On the 

 western coasts of the Atlantic, from the Bahamas north- 

 wards to Newfoundland, the temperature is at the annual 

 maximum, but the air is not dry, being liberally supplied 

 with moisture, and the rainfall is generous. But with 

 these very different meteorological conditions there occurs, 

 equally as in Southern Europe, a diminished oscillation 

 during the summer months in the islands and near the 



' Possibly some coasts may furnish an exception. 



