530 



Captain Spratt on the Undercurrent 



[June 15, 



In August 1848 I read a paper at the Swansea Meeting of the British 

 Association*, " On the Influence of Temperature upon the Distribution 

 of the Fauna in the iEgean Sea," as an explanation of the zones 

 of animal life in that sea, which had been discovered by Edward Forbes 

 a few years previously, when we were employed together in the 'Beacon/ 

 I then showed that the temperature in depths below about 100 fathoms, 

 although sometimes in midsummer trenching down to between 200 and 

 300 fathoms, was always uniform, the minimum of 55J° Fahr. (as 

 the thermometers then gave) being reached at that depth ; and this 

 fact I had discovered in 1845 to exist in both divisions of the Medi- 

 terranean, although in the latter the minimum was not so low as in the 

 iEgean by about 3° or 3^°. As the thermometers in use at that time 

 were defective in construction for such observations by (as now found) a 

 constant error in excess of about 3|° or 4°, the temperatures, with this de- 

 duction made from them, agree with the recent observations of Dr. Car- 

 penter. Also, in a paper published in the £ Nautical Magazine 5 for January 

 1862, "On the proper Depths for Deep-sea Cables," as the result of my 

 experience gained after conducting the laying of the Varna and Crimean 

 Cables across the Black Sea, and others in the Levant, and also between 

 Malta and Alexandria, when the temperatures were constantly taken, I 

 stated the following interesting facts. 



Extract from the ( Nautical Magazine? January 1862. 



"The Mediterranean temperatures are known to be not very low at 

 great depths, but reach their minimum as a permanency in from 100 to 

 300 fathoms ; and this minimum temperature seems to correspond with 

 the average annual temperature of the locality itself. And as the Medi- 

 terranean is divided into a series of basins, with comparatively intermediate 

 shallows, it is its surface waters, about the depth of 200 or 300 fathoms 

 (being that of the barriers which separate them), that unite by their 

 superficial and encircling currents. Thus, as the depth across the Strait 

 of Gibraltar is under 200 fathoms, the very cold waters in the deeps of the 



* British Association Eeport, 1848 (Swansea Meeting), Sections, p. 81. Also, see 

 Edinburgh Philosophical Journal, 1848, in which are the following remarks: — 



" It is temperature, and local conditions partially arising from it, that limits the 

 elevation and existence of animal life. So does -the same law appear applicable to 

 marine animals, which breathe the medium they inhabit. 



" As a law resulting from this influence, characteristic, tropical, and subtropical species 

 will have a limited distribution in geographical space, whilst the boreal and sixbboreal 

 characters will be found in every geographic position, where corresponding regions of 

 depth are found with animal life existing, the limit of which I believe to be much lower 

 than 300 fathoms, having examples from 390 fathoms. But I must notice that the 

 iEgean deep dredges indicate generally a zero of animal life at 300 fathoms, as Professor 

 Forbes was induced to assume. I believe, however, that in the deserts of yellow clay 

 an occasional oasis of animal life may be found in much greater depths, dependent 

 upon some favourable local condition or accident." 



