xxx INTRODUCTION. 
three days of almost incessant rain. In winter, the showers also occasionally fall as 
snow on the higher mountains, such as Jebel Kittar, covering them with a mantle of 
white and their sides with a network of streams 1 . 
The variations of temperature to which the Arabian desert is subject are very con- 
siderable, being as much as 30° to 35° Centigrade in the 24 hours, whilst, on the 
heights, frost is not unfrequent during winter. The cold of the winter months exercises 
a powerful influence on animal life, rendering many species, more especially reptiles, 
almost semidormant while it lasts. It is only when the sun fully exercises its sway in 
the months of summer that reptiles may be said to become possessed of their full vital 
activity. This is also true, but to a more limited extent, of the smaller mammals. 
Some of the latter which I have kept in confinement, although they were given every 
protection in the way of a covering during night, were found in the early morning so 
utterly benumbed with cold as to give rise to the impression that they were in a fatal 
collapse, but when the sun rose, and they were exposed to its heating influence, their 
powers of life returned to them. I have observed this both in the Insectivora and 
Rodentia, and, as already stated, even camels, in the early morning, are occasionally so 
torpid from the benumbing action of the cold, that the hour of departure of a march 
has to be postponed until they become revivified by the sun. 
The region on the left bank of the Nile known as the Libyan desert has now briefly 
to be considered, so as to bring out the conditions under which animal life subsists 
upon its surface. Its physical features differ considerably from those of the Eastern 
desert, as it is not traversed by a mountain-ridge, and is consequently devoid of any of 
the great altitudes met with to the east of the Nile. It is also not cut up into deep 
transverse valleys and ravines, but forms a great tableland with a gradual, but in 
places irregular, slope from south to north, and attaining on an average to an altitude 
of from 250 to 300 metres, although rising in parts to 500 metres above the sea. 
Depressions of varying size and depth are met with over its surface, some of them 
not more than a few miles in length and breadth, while others are hundreds of 
square kilometres in extent. They are all more or less enclosed by bold escarpments 
resembling those which define the plateau where it skirts the Nile, and in some of them 
the descent to the oasis is made through labyrinths of fantastic rocks. The floors of 
these depressions, the sites of the famed oases, lie at various heights, but there is no 
regularly graduated slope from south to north, as the floor of the oasis at Farafreh 
(76 metres above the sea) lies at a lower level than that of Dakhcl (100 metres), 
whereas that of Baharieh (114 metres) is somewhat more elevated than that of Dakhel, 
that is, there is a distinct fall from Dakhel to Farafreh, and a marked rise from the 
latter to Baharieh, beyond which the land sinks, in the Aradj and Siwah district, to 
70 metres and 30 metres below the level of the Mediterranean. The famous Birket-el- 
1 Floyer, Proc. Eoy. Geogr. Soc. 1S87, p. 678. 
