118 



KNOWLEDGE 



[June 1, 1892. 



the Crimea to the north-west, and then follows the trend 

 of western shores past the mouths of the Danube towards 

 the Bosphorus. The dimensions and velocity of this cur- 

 rent are augmented when the melting of the snow in the 

 Black Sea basin is more rapid than usual. 



We have mentioned above that a decided influx of 

 Mediterranean water takes place into the Black Sea. Were 

 it not for this saline water the Black Sea would be much 

 fresher than it is, and were the connection between it and 

 the Mediterranean destroyed then the sea would become 

 fresher, as there is a surplus of river and rain supply over 

 evaporation. Throughout the whole length of the Bos- 

 phorus, the Sea of Marmora, and the Hellespont, two 

 distinct currents can be traced, the heavier Mediterranean 

 water forming the underlying stratum, moving slowly into 

 the Black Sea, the lighter water from the latter sea being 

 superincumbent and moving in the opposite direction. 

 There appears to be very little mixing of the two currents, 

 the layer of demarcation between the two being easily de- 

 tected by the diti'erence in the specific gravities of the two 

 aqueous masses. 



The comparatively fresh water that the Black Sea con- 

 tributes to the waters of the Mediterranean produces but 

 little effect, so large is the area of the basin into which it 

 pours itself and so small relatively is the volume of water 

 so contributed. 



The greatest depth of the Sea of Marmora is found along 

 the line connecting the Hellespont with the Bosphorus, 

 and ranges between 26G and 3.55 fathoms. 



The Hellespont itself has a depth of 50 fathoms, while 

 the mean depth of the Bosphorus varies from 30 to 40 

 fathoms. 



The ^Egean Sea has not its specific gravity reduced as 

 might be expected below that of the whole Mediterranean 

 by the influx of the Black Sea water. In fact its specific 

 gravity is greater than that of any other section of the 

 Mediterranean basin. 



The accompanying table bears out the truth of this. 

 The figures quoted are the result of many observations, 

 and are derived from samples of water taken from 60 

 miles to the westward of Gibraltar to the easternmost 

 section of the Black Sea. 



Mediterranean Water outside ') , .n.^ar, i nn^-n 



Gibraltar ) ^ ""^^ ~ ^ ^^/O 



Mean specific gravity for Western ) i(|-?^n 1-0290 



Section Jl 



Mean specific gravity for Eastern > i.o.9i,|j _ 1U30O 

 Section j 



Mian specific gravity for Black ) iol20 -— 10140 

 Sea ) 



In each case the results are derived from analyses of 

 surface water. 



In round numbers the area of the Mediterranean basin 

 is one million square miles, and the average rain-fall over 

 the whole area that drains into it has been assessed as 

 being equivalent to an annual rain-fall of 30 inches upon 

 the sea itself. The amount of water removed by evapo- 

 ration is greatly in excess of this, probably 2i times as 

 great. At Eome the evaporation is represented at some 

 105 inches per annum, at Madrid it is 65 inches, and at 

 Cairo 92 inches. It does not require much speculation to 

 determine what would happen if the waters of the Mediter- 

 ranean were not replenished from some external source. 

 A shrinkage of the basin by a diminution of the water 

 area would continue until the evaporation from the 

 reduced surface would equal the amount of aqiteous 

 precipitation. But before that condition would be reached, 

 the shrinkage would have resulted in the drying of the 

 bank between Sicily and Africa, and between Africa and 



Gibraltar, with the result that two " dead" seas would be 

 formed. 



Happily, there is not much probability of such a change 

 taking place, for the Mediterranean is supplied with water 

 from the Atlantic as well as the Black Sea. Through the 

 Straits of Gibraltar there flow two currents, as there do 

 through the Sea of Marmora. The existence of an out- 

 ward current has been long known. It is accountable for 

 the higher temperature found in the deep waters of the 

 Eastern Atlantic. Water, unmistakably of Mediterranean 

 origin, has been found some 200 miles north-west of the 

 Straits at a depth of 15G0 fathoms. Its presence at such 

 a depth is readily understood when it is remembered that 

 the water of the Mediterranean contains an average of 

 3-9 to i per cent, of solid matter in a state of solution, 

 while the percentage in Atlantic water in the vicinity of 

 the Mediterranean entrance is 3-4 to 3'5. 



It has been calculated that the injiow through the Straits 

 is equivalent to a river eight miles wide, 100 fathoms deep, 

 running with an uniform velocity of IH^ miles in the 

 twenty-four hours. Such are the dimensions of a current 

 requisite to maintain equilibrium between the contribution 

 of rivers, precipitation supply, and the inflow from the 

 Black Sea on the one hand, and the copious evaporation 

 and the outflow into the Black Sea and the Atlantic 

 Ocean on the other. 



THE FACE OF THE SKY FOR JUNE. 



By Herbert Sadler, F.R.A.S. 



AT the time of writing these lines several flue spots 

 and groups are visible on the solar surface. 

 Throughout June there is no real night, but either 

 daylight or twilight. A minimum of the Algol- 

 type variable U Coronse will occur at lOh. 11m. 

 P.M. on the 20th. 



Mercury is technically a morning star during the first 

 portion of the mouth, but is in reality too near the Sun to 

 be observed. He comes into superior conjunction on the 

 20th. Venus, during the greater portion of the month, is 

 tlie conspicuous object in the evening sky. She attains 

 her greatest brightness on the 4th, when she is more than 

 three times brighter than she was on January 1st. She 

 sets on June 1st at llh. 14m. p.m., or 3h. 9m. after the 

 Sun, with a northern declination of 24° 16', and an 

 apparent diameter of 36", i%%ths of the disc being 

 Illuminated. On the 9th she sets at lOh. 45m. p.m., or 

 2h. 34m. after the Sun, with a northern declination of 

 22° 49', and an apparent diameter of 40-8", I'^ths of the 

 disc being illuminated. The crescent form of the planet 

 may now be easily seen in a good opera or field glass, if 

 Venus is viewed before sunset or just after, so that her 

 glare does not interfere with distinct vision. On the 17th 

 she sets at lOh. 10m. p.m., or Ih. 53m. after the Sun, with 

 a northern declination of 21° 18', and an apparent diameter 

 of 46|-", about yVotl^s of the disc being illuminated. On 

 the 24th she sets at 9h. 29m. p.m., or Ih. 10m. after the 

 Sun, with an apparent diameter of 51|", y^otlis only of 

 the disc being illuminated, while her brightness has fallen 

 to one-half of what it was on Jtuie 4tb. After this date 

 she IS too near the Sun to be clearly seen. During the 

 month she hovers on the confines of Gemini and Cancer ; 

 but does not approach any bright fixed star. 



Mars is an evening star, but owing to his great southern 

 declination is badly placed for observation in these latitudes. 

 He sets on the 1st at Oh. 3m. a.m., with a southern declina- 

 tion of 20° 23', and an apparent diameter of llf", the 

 phase on the sottth-western limb amounting to 1-5", and 



