236 NATURAL SCIENCE. Oct., 



Into the mesoglcea hollow outgrowths of the endoderm grow, and form 

 a definite system of cavities. Externally to this, the layer of 

 mesoglcea is formed by intruding cells from the ectoderm. Outside 

 this again another definite layer of cells of ectodermic origin forms a 

 well-marked layer of what Mr. Bourne calls calicoblasts, and it is this 

 layer that gives rise to the skeleton. An examination of Xenia and 

 Hcteroxenia led Mr. Bourne to think that probably in a large number 

 of alcyonarians the skeleton is ectodermal. 



The matter on which we have touched is only a small part of 

 Mr. Bourne's memoir. He discusses at length the nature of Heliopora, 

 of the "animal" and the coral; but these are subjects too technical to 

 be summarised here. 



The Temperature of the River Nile. 



We have received from Dr. H. B. Guppy a reprint from the 

 Proceedings of the Royal Physical Society of Edinbuvgh, containing Part II. 

 of his work on River Temperatures. This second instalment deals 

 chiefly with the temperature of the waters of the Nile and its relation 

 to the temperature of the air at different points and in different 

 seasons, and is obviously the result of considerable labour and 

 research. Observations of the temperature of the Nile are remarkably 

 scarce, the pencil manuscripts of Mr. Robert Hay, which Dr. Guppy 

 has unearthed in the British Museum, containing one of the most 

 valuable series extant. The leading conclusions of the paper are 

 accordingly based on comparisons with air temperature rather than 

 on the actual water temperatures, a method which has recently been 

 found very successful. We cannot do better than give the pro- 

 positions stated by Dr. Guppy and ably supported by the discussion 

 of the observations. During its course below the second cataract the 

 Nile is in summer markedly cooler than the air ; but the relative 

 coolness diminishes as the river flows north, and the diminution is 

 almost entirely due to the fall in the air temperature. In winter the 

 Nile is still colder than the air between the first and second cataracts, 

 but the difference of temperature is much less : between Assouan and 

 Minieh the water is nearly two degrees (Fahr.) warmer than the air, 

 and the excess is greater at Cairo. The approximation and crossing 

 of the air and river temperatures is again mainly due to the fall of air 

 temperature. It is remarkable that during the four seasons there is 

 no great contrast in temperature between the water of the Nile and 

 the surface waters of the Mediterranean. 



Dr. Guppy further compares the thermal regime of the Nile with 

 those of other great rivers. While pointing out that further acquain- 

 tance with the Brahmaputra and the Yangtse will probably disclose 

 points of similarity, Dr. Guppy shows that the Nile is in strong 

 contrast to the Congo and the Amazon, which flow east and west, 

 and to the Mississippi, flowing south. The comparison with the last- 



