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THE POPULAR SCIENCE MONTHLY. 



is now ; and I believe that the conquest of 

 Egypt by the Hyksos is not unlike what 

 would happen at the present day if the 

 population of Mesopotamia overran the val- 

 ley of the Nile : you would have masses, in 

 great majority of Shemitic race, speaking a 

 Shemitic language, and having a Shemitic 

 religion, but under the command of Turks, 

 who are not Shemites, but Turanians." M. 

 Naville regards the successive discoveries 

 that have been made in the Delta as making 

 the Bible-story more comprehensible in some 

 points, and as showing that the distances were 

 much shorter than was generally thought. 

 "I consider it important, for instance, to 

 have established that Bubastis was a very 

 large city, and a favorite resort of the king 

 and his family. It is quite possible that, at 

 the time when the events preceding the Ex- 

 odus took place, the king was at Bubastis, 

 and not at Tanis, as has been generally be- 

 lieved." 



Composition of London Fogs. Studies of 

 London fogs by various observers show that 

 during the winter the air of the metropolis has 

 an unusually large amount of carbonic acid 

 in it. Thus, Dr. W. J. Russell found on one 

 day, a few years ago, that it contained more 

 than three and a half times the average 

 amount. This is derived, to a large extent, 

 from respiration, and more from coal-burn- 

 ing ; and " it is almost indisputable that the 

 latter produces the well-known black fogs 

 and yellow fogs." The relative thickness and 

 density of the air of different parts of London 

 have been investigated by Mr. W. H. Raffles, 

 who took a station on Primrose Hill and ob- 

 served the visibility, on different days, of 

 prominent objects at known distances in dif- 

 ferent directions. These observations showed 

 plainly that the amount of fog was largely 

 governed by the density of the population 

 and the frequency of factories. A similar 

 conclusion is drawn from the number of 

 hours in the daytime on which artificial light 

 was used. Ilomerton had twice as many 

 hours of darkness as any other district rep- 

 resented in the tables, and it has a very 

 large number of factories in its neighbor- 

 hood. Of other towns in which the inquiry 

 was pursued, Leeds suffered most from dark- 

 ness, probably for the same reason ; while 

 Manchester is said to have been unusually 



free of late years from dark fogs, probably 

 because many mills have moved out. The 

 ordinary white fog has also been reduced by 

 the draining of morass lands near the city. 

 A conception of the cost of London fogs 

 may be gaiued from the fact that during 

 nine days of fog in November, 18S7, the 

 public paid a single one of the several com- 

 panies four hundred and ninety pounds, or 

 twenty-four hundred and fifty dollars, an 

 hour for artificial light. 



Geological History of Yellowstone Park. 



The geological history of Yellowstone Na- 

 tional Park has been traced by Mr. Arnold 

 Hague in an address before the American 

 Institute of Mining Engineers. Throughout 

 Tertiary time the history was characterized by 

 great volcanic activity. Within very recent 

 times there is no evidence of any consider- 

 able outburst; indeed, the region may be 

 considered long since extinct. The volcanic 

 rocks present a wide range in chemical and 

 mineral composition and physical structure, 

 but may all be classed in the groups, follow- 

 ing one another in the order named ande- 

 sites, rhyolites, and basalts. Since the close 

 of the Ice period no geological events of any 

 moment have brought about any changes in 

 the physical history of the region other than 

 those produced by the direct action of steam 

 and thermal waters. Indications of fresh 

 lava-flows within historical times are wholly 

 wanting. All our observations point in one 

 direction, and lead to the theory that the 

 cause of the high temperature of the waters 

 of the geysers and hot springs must be 

 found in the rocks below, and that the ori- 

 gin of the heat is in some way associated 

 with the source of volcanic agency. But it 

 does not follow that the waters themselves 

 are derived from any deep-seated source; 

 on the contrary, investigation tends to show 

 that the waters brought up by the springs are 

 mainly surface-waters which have percolated 

 downward a sufficient distance to be heated 

 by large volumes of steam ascending through 

 fissures and vents from much greater depths. 

 The existence of such currents of steam and 

 hot water is attested by the decompositions 

 they have effected upon the rocks, which 

 " have proceeded on a most gigantic scale"; 

 and they have left an indelible impression 

 upon the surface of the country. The study 



