stir and mix, and your picture is finished.  Most practical deserts one comes across in travelling, however, are a great deal less simple and theatrical than that; rock preponderates over sand in their composition, and inequalities of surface are often the rule rather than the exception.  There is reason to believe, indeed, that the artistic conception of the common or Burlington House desert has been unduly influenced for evil by the accessibility and the poetic adjuncts of the Egyptian sand-waste, which, being situated in a great alluvial river valley is really flat, and, being the most familiar, has therefore distorted to its own shape the mental picture of all its kind elsewhere.  But most deserts of actual nature are not all flat, nor all sandy; they present a considerable diversity and variety of surface, and their rocks are often unpleasantly obtrusive to the tender feet of the pedestrian traveller.

A desert, in fact, is only a place where the weather is always and uniformly fine.  The sand is there merely as what the logicians call, in their cheerful way, ‘a separable accident’; the essential of a desert, as such, is the absence of vegetation, due to drought.  The barometer in those happy, too happy, regions, always stands at Set Fair.  At least, it would, if barometers commonly grew in the desert, where, however, in the present condition of science, they are rarely found.  It is this dryness of the air, and this alone, that makes a desert; all the rest, like the camels, the sphinx, the skeleton, and the pyramid, is only thrown in to complete the picture.

Now the first question that occurs to the inquiring mind—­which is but a graceful periphrasis for the present writer—­when it comes to examine in detail the peculiarities of deserts is just this:  Why are there places on the earth’s surface on which rain never falls?  What makes it so uncommonly dry in Sahara when it’s so unpleasantly wet and so unnecessarily foggy in this realm of England?  And the obvious answer is, of course, that deserts exist only in those parts of the world where the run of mountain ranges, prevalent winds, and ocean currents conspire to render the average rainfall as small as possible.  But, strangely enough, there is a large irregular belt of the great eastern continent where these peculiar conditions occur in an almost unbroken line for thousands of miles together, from the west coast of Africa to the borders of China:  and it is in this belt that all the best known deserts of the world are actually situated.  In one place it is the Atlas and the Kong mountains (now don’t pretend, as David Copperfield’s aunt would have said, you don’t know the Kong mountains); at another place it is the Arabian coast range, Lebanon, and the Beluchi hills; at a third, it is the Himalayas and the Chinese heights that intercept and precipitate all the moisture from the clouds.  But, from whatever variety of local causes it may arise, the fact still remains the same,