116 



KNOWLEDGE 



[June 1, 1892. 



((') Large, woolly stars, with ill-defined image, resem- 

 bling a small but bright planetary nebula. 



(d) Stars which at minimum show, in place of the 

 variable, a slight bluish nebulosity. 



In the case of a star surrounded by a very faint nebu- 

 losity or coronal appendage, we should not expect to see any 

 trace of the corona till the star was nearlj' or totally 

 obscured, and recent observations show that in several 

 instances, when the star has gradually become so faint as 

 to be invisible with our G^^, inch refractor, its place has 

 been occupied by a pale bluish nebulosity, which has again 

 vanished as soon as the star reappeared, being evidently 

 so faint as to be readily overpowered by a small amount of 

 stellar light. 



It may be that all variables are nebulous, or have 

 extensive and bright coronas, but that when the nebulosity 

 is faint, we only become aware of its existence when the 

 light of the variable is reduced to a minimum. 



Yours faithfully, 



Eousdon Observatory, Lyme. C. Grover. 



THE DEPTHS OF THE MEDITERRANEAN AND 

 BLACK SEAS. 



By EicH.\ED Beynon, F.E.G.S. 



ISOLATED as it is from the great water masses of the 

 globe, the Mediterranean — with its off-set, the Black 

 Sea — may be regarded as a provincial sea. The 

 oceanic circulation of the North Atlantic sweeps past 

 its narrow entrance imheeded. The great tidal wave 

 is effectually debarred by the convergence of the African 

 and European coasts from influencing the tidal phenomena 

 of the Mediterranean, and the same cause, aided by the 

 near approach of the strata underlying the Straits of 

 Gibraltar to the surface, precludes the possibility of the 

 chill waters that ever roll equatorwards along the sea floor 

 finding their way into the vast inland sea under discussion, j 

 The geographical limits of the Mediterranean are well i 

 known, but its true geological boundaries by no means ; 

 coincide with these. Instead of terminating to the west- 

 ward at the Straits of Gibraltar, the sea is really continued 

 some 50 miles into the Atlantic, for the shoal water which 

 separates the line of coast between Tangier and Ceuta from 

 the opposite shores of Spain extends westwards to that 

 distance. Here the shallow ridge terminates, and the 

 sea bed rapidly falls into the depths of the Atlantic. 



The proximity of the island of Sicily to Cape Bon 

 suggests the very natural division of the Mediterranean 

 into an eastern and western section. Taking the western 

 portion, we find that at its two lateral extremities it is 

 separated by a shallow ridge from the Atlantic on the one 

 side, and the deep waters of the Eastern Mediterranean on 

 the other. The depths of water obtaining on these 

 shallows approximately coincide. The deepest sounding 

 obtainable on the ridge between Cape Bon and Sicily is 

 under 200 fathoms, while the maximum depth in the 

 vicinity of Gibraltar is 180 fathoms. 



The shoal water which commences some 50 miles to 

 the westward of Gibraltar is really continued 120 miles to 

 the eastward of Point Europa, almost to the shores of | 

 Alboran Island. i 



If we regard as continental, islands that are separated 

 from the mainland by depths not exceedmg 100 fathoms, 

 then Alboran must be classed as an oceanic island, for on 

 all sides it is surrounded with water of 400 fathoms and 

 upwards in depth. The Balearic group present most 

 peculiar features. Instead of formmg one group, as their 



juxtaposition would seem to imply, they make two. Iviza 

 and Formentara are separated from Majorca, Minorca, and 

 the Spanish coast by soundings of 300 fathoms. The two 

 last named islands have a channel of 50 fathoms between 

 them, and to the eastward of Minorca the sea bed has a 

 steej) gradient until, 30 miles from the Balearic group, a 

 depth of 1400 fathoms is encountered. A comparatively 

 slight upheaval of the Mediterranean bed would suffice to 

 connect Corsica and Sardinia, for the Straits of Bonifacio 

 are of little depth. Shoal water, too, connects this group 

 with Elba and the mainland of Italy. The shallow 

 channel which extends from Cape Corso, rii': Elba, to the 

 coast of Tuscany has an average width of from 15 to 20 

 miles, and nowhere along it can soundings of a greater 

 depth than 50 fathoms be obtained. 



We now come to the easterly boundary of the western 

 portion of the Mediterranean Sea. 



From Cape Passaro, at the south-easterly corner of 

 Sicily, a bank with 300 fathoms of water over it extends 

 to the opposite shores of Tripoli, while a somewhat similar 

 ridge, with a lesser depth of 200 fathoms, connects the other 

 extremity of the island with Cape Bon. Between these 

 two banks a deep water gully runs, with an average depth 

 of GOO to 700 fathoms. 



With regard to the deep water areas of the western section 

 of the Mediterranean, a fairly uniform depth, ranging 

 between 1200 and 1600 fathoms, is maintained between 

 Marseilles and Algiers, while the deep water lane extending 

 from Naples to Sardinia admits of soundings of 1500 to 

 2000 fathoms. 



These are the more salient features revealed by sound- 

 ings taken in the western portion of the Mediterranean. 

 Scientific research, however, has added much to our know- 

 ledge of the eastern section during the past few years, and 

 it is chiefly to Austria that progress in the study of the 

 oceanography of this part of the Mediterranean is due. 



Before the Pala expedition the generally received greatest 

 depths obtained in the Mediterranean were 2040 fathoms 

 in the western section and 2150 in the eastern. The 

 latest results, however, show that deeper soundings are 

 obtainable. On the 28th July, 1891, the Pula found the 

 depth of 2406 fathoms, and a few miles further to the 

 eastward 2236 fathoms, both of which depths exceed those 

 mentioned above. The exact position of this, the deepest 

 spot yet discovered, is 35° 44' 20" north lat., and 21'^ 44' 50" 

 east long., or, roughly speaking, about 50 nautical miles 

 south-west of Matapan. Very properly the Austrian 

 Ilydrographical Board have determined to perpetuate the 

 record of their nautical find by assigning to this deep- 

 water spot the name of Pola Deep. This discovery will 

 necessitate the removal of the deepest part of the Medi- 

 terranean considerably eastwards from its present position 

 on our maps. Another deep-water area explored by the 

 Pola was that lying between Candia and Alexandria, the 

 depths ranging from 1810 fathoms, some 20 miles south- 

 east of Grandes Bay, to 1822 fathoms within a short dis- 

 tance of Alexandria. The serial temperatures taken by 

 this expedition coincide in the main with those obtained 

 during previous researches. 



From 80-8° F. to 69° F. was the thermometric range in 

 the first 27 fathoms. In the next 27 fathoms the tem- 

 perature fell to 625° F. The range for depths between 

 110 and 547 fathoms was 59° F. to 57° F. At the lowest 

 depth found (2406 fathoms), the temperature was 56° F., 

 which, as previous investigators have established, is the 

 approximate uniform temperature of the bed of the Medi- 

 terranean. One very curious result of the temperature 

 experiments was the finding of water whose temperature 

 was 52-^° P\, at a depth of 415 fathoms, at the junction of 



