204 



T)ed of the canal. Arundo pliragmites, gnaplialium, and a species 

 of lotus appear to prevail. At Isinailia I went ashore by spe- 

 cial arrangement, and on the shores of Lake Timsah I obtained 

 many specimens of marine mollusca, including Gardium edule, 

 Mactra solida, and Mytilus, which the Arabs affirmed had mi- 

 grated from the Eed Sea. 



The maximum air temperature during the passage of the 

 Canal was 77*3, and the minimum 61*0°. The barometer, fully 

 reduced, ranged between 30080 and 29*904 during the passage 

 •of the Canal. 



I may mention that the highest sea surface temperature yet 

 observed is 94° near the Strait of Bab-el-mandeb. On looking 

 at the synopsis of this section it will be seen that 88'9° was the 

 highest observed on this passage in lat. 17° 45' N. The mean 

 temperature of the Eed Sea water north of latitude 20° is 77° 

 4', and south of that parellel 81° 5'. In many parts of the Eed 

 Sea the water is intensely blue, the colour being quite as deep 

 as in the Mediterranean. I observed this more particularly in 

 lat. 26° 20' N". The cause of this blue colour appears as yet to be 

 imperfectly understood, and probably will remain so until the 

 perfection of a sub-aqueous exploring telescope, as suggested 

 by Sir J. F. W. Herschel, Bart., in the fourth edition of the 

 "Admiralty Manual of Scientific Enquiry," has been realised. 



At Aden I secured several specimens of metamorphic slates 

 and calcite with some pieces of tracytic or felspathic lava. 



We sailed from Aden on November 8th as the north-east 

 monsoon was becoming established, and on 10th we passed 

 Socotra. an admirable position for a high-level meteorological 

 station by the way, beiug 4,656 feet high. In this neighbour- 

 hood I observed many fine specimens of Acalephce, purple 

 "jelly-fish," probably the JElquorea purpurea. They abounded 

 in great numbers. JExocietas volitans was also prevalent. 

 Here again the water was of a deep blue colour. Intense vapour 

 bands w r ere observed during this section to the left of the D 

 line in the spectrum at 7° above horizon as in the Mediter- 

 ranean and Eed Sea. 



The prevailing upper wind current was N.E. by E., the clouds 

 travelling with a velocity of about 25 on a scale of to 10. 

 The skies of the north-east monsoon are truly splendid, and the 

 cloud-forms often exceedingly curious. One instance only I 

 can mention. On November 14th, after sunset, against a golden 

 background, rose as from the offing dark cumuli of most gro- 

 tesque shapes. They resembled trees, branches, islands, rocks, 

 and even solar prominences, with heavy cumulo - stratus 

 stretched above, while the full moon at the opposite azimuth, 

 •on a sky of grey blue and reflected on the water, added charm 

 to the picture. I merely mention this because many travellers 



