70 



NATURE 



[July 15, 1909 



the heated face tends to retreat with a force propor- 

 tional both to the density of the gas and to the area 

 of the surface. 



A calculation of the absolute value of the excess of 

 pressure cannot be made without further' hypothesis. 

 If we were to suppose that the molecules, after col- 

 lision with the heated face, rebound with the same 

 velocities (I'+tii') as they would have were the tempera- 

 ture raised throughout, the pressure would be in- 

 creased in the ratio ■v -V (u -k- dv) : zv or i + dv/zv : i. 

 On the other hand, if the temperature were actually 

 raised throughout, the pressure, according to the usual 

 gaseous laws, would be increased in the ratio 

 (v+dv)- : V- or i + 2dvjv : i. On this hypothesis, there- 

 fore, the unbalanced increment of pressure on the 

 heated face is one-quarter of the increment that would 

 be caused by a general rise of temperature to the 

 same amount. This estimate is necessarily in excess 

 of the truth, but it is probably of the right order of 

 magnitude. 



The supposition upon which our reasoning has been 

 based, viz. that the mean free path of a molecule is 

 large in comparison with the linear dimension of the 

 vessel, has been made for the sake of simplicity, and 

 is certainly a very extreme one. It is not difficult to 

 recognise that in the extreme form it may be dispensed 

 with. All that is really necessary to justify our con- 

 clusions is that the mean free path should be very 

 large in comparison with the vane. The magnitude 

 and distribution of the velocities with which the 

 molecules impinge will then be independent of the 

 fact that the face of the vane is heated, and this is 

 all that the argument requires. The repulsion by 

 heat of a silk fibre suspended in a moderately rare 

 gas was, it will be remembered, verified by O. 

 Revnolds. Rayleioh. 



LIFE IN AN O/lS/S.i 

 A LTHOUGH the oases of the Libyan Desert have 

 ■^~*- been frequently visited by travellers — Poncet in 

 the seventeenth century, Browne in the eighteenth 

 century, and Cailliaud, Drovetti, Edmonstone, 

 Hoskins, Rohlfs, Zittel, Schweinfurth, Brugsch, and 

 others in the nineteenth century — yet none of these 

 authors enjoyed anything like the opportunities 

 for the study of these remarkable districts which have 

 fallen to the lot of the writer of the work before us. 

 For nine years Mr. Beadnell, as a member of that 

 active body the Egyptian Geological Survey, was 

 engaged in the study of the Libvan Desert — including 

 the four oases of Baharia, Farafra, Dakhla, and 

 Kharga — while during the last three vears, as director 

 of the operations of a development company, he has 

 resided in the last mentioned, and has carried out 

 important observations and experiments in connection 

 with the questions of water supply, the effects of 

 moving masses of sand in increasing the fertility of 

 some • areas, while overwhelming and destroying 

 others, as well as of many other problems of great 

 historical and antiquarian interest. 



Now that the opening of a railwav from Oena, a 

 little north of Luxor, to the village of Kharga has 

 been completed, the long and tedious camel-journev of 

 four or five days along very rough caravan routes is 

 avoided, and excursions from the Nile valley to this 

 typical oasis will doubtless become much more fre- 

 quent. The appearance of the present work is, there- 

 fore, very opportune. The detailed topographical and 

 geological survey of the Libyan Desert with its oases 



* " An Ecypt'an Oasis : an Account of the Oasis of Kharga in the Libyan 

 Desert, with special reference to its History, Physical Geofrraphy, and 

 Water Supply." By H. J. Llewellyn Beadnell, Pp. X + 24S ; wiih 28 plates 

 and 4 maps and sections. (London : John Murray, 1509.) Price los. 6</. 

 net. 



NO. 2072, VOL. 81] 



was undertaken in 1847-8. Mr. Beadnell carried out 

 the mapping of the Farafra and Dakhla o.ises, while 

 Dr. Ball was engaged in surveying that of Kharga, 

 the work in the Baharia Oasis being shared between 

 the two investigators. Dr. Ball's map of tiie Kharga 

 Oasis, with the accompanying official report, is a 

 w-ork of great geological value and interest, and Mr. 

 Beadnell's residence in the district has enabled him to 

 add not a few important scientific details to the 

 admirable sketch given by his colleague. 



The whole Libvan Desert forms a plateau, having 

 an elevation which, at its maximum, is but little less 

 than 2000 feet above sea-level, yet with a fairh' general 

 slope towards the north. In this great expanse of 

 rough limestone and flint-covered flats, with 

 hillocks and troughs of drifting sand, the oases are 

 deep depressions, the bottoms of which vary from 

 100 to 300 feet above sea-level, surrounded, for the 

 most part, by steep escarpments, through which only 

 a few passes can be found which are capable of being 

 used as camel-tracks. The whole of the deserts are 

 underlain by great beds of sandstone (the Nubian 

 series), forming two divisions, the " surface-water 

 sandstones," never more than 160 feet thick, separated 

 by 250 feet of impervious grey shales, from a much 

 thicker series of sandstones below, the " artesian- 

 water sandstone,"' which has been penetrated bv 

 borings to the depth of 400 feet. 



It is by the removal, through denudation, of great 

 masses of Eocene and L'pper Cretaceous limestones 

 and shales that the " surface-water sandstones " have 

 been exposed on the floors of the oases. These beds 

 are the source of springs, and, since the districts have 

 been occupied by human beings, a ^reat part of the 

 area of the Kharga Oasis was covered by shallow 

 lakes, probably formed by the outflow from these 

 springs. But these great lakes have been gradually 

 dried up, and the constant drain on the limited supplies 

 of water afforded by the " surface-water sandstones " 

 has greatly reduced its importance as a means of 

 irrigation. The accounts of the various deposits laid 

 down in these old lakes, with their interesting con- 

 tents of w'orked flint-flakes and pottery, are among 

 the most novel and interesting portions of Mr. 

 Beadnell's book. 



Far more important, however, than the surface- 

 water sandstones, as a source of irrigation water, 

 are the " artesian-water sandstones," which, by 

 means of borings, have been drawn upon from the 

 earliest times, and constitute even now ;i practically 

 inexhaustible means for promoting the cultivation of 

 the oases. On all questions connected with the nature 

 and amount of the yield of the different kinds of 

 wells, the author of this book writes as an authority, 

 and he is able to give the results of numerous in- 

 genious experiments, carried on, in some instances, for 

 many months. That the enormous quantities of water 

 contained in the thick sandstones of the Nubian 

 system have their source, in part in the highlands of 

 Abyssinia, in part in the Sudan, and to some extent 

 in the upper waters of the Nile, where it flows over 

 these pervious sandstones, there can be little doubt, 

 though as to the proportional parts played by these 

 .several factors of the supply there is still much room 

 for doubt — a doubt which can only be removed by 

 prolonged obser\'ations. 



The manner in which the ancient wells have been 

 made, kept open, and from time to time repaired, has 

 engaged the author's attentive study. It is surprising 

 to learn how much has been acconiplished with the 

 aid of very simple appliances ; and the long sub- 

 terranean aqueducts — tunnels driven for miles into 

 the sandstones for the purpose of increasing the flow 

 of water — with numerous manholes up to tlie surface. 



