28 
OPTICAL LAKE. — FRESHWATER HALT, 
2 Mabch, 
as we advanced ; and offered a prospect the most tempting and de- 
lightful, but at the same time the most tantalizing, to a thirsty 
traveller ; and, to complete the illusion, its surface seemed to play 
in a lively rippling motion. The day was warm and cloudless. The 
cause of this phenomenon is, very probably, the vapor and heat 
of the sun's rays, reflected from a great extent of level surface. 
A similar effect, on a small scale, may be observed even in England, 
on very hot days : not asserting that it ever produces the appearance 
of water ; but meaning only that the operation of the same cause 
may often be witnessed, by looking obliquely along any heated and 
extensive surface. In the present example of this phenomenon, and 
in all others which I have seen in Africa, the optical lake is only 
visible when backed by very distant mountains, whose angle of alti- 
tude is not greater than ten or fifteen minutes of a degree ; that is, 
just appearing above the horizon : for, as the traveller advances, 
and these mountains appear higher, the lake always vanishes. Con- 
sequently, it divides itself into separate lakes or ponds, as soon as the 
spectator's nearer approach occasions the higher mountains of the 
range, or the loftier peaks, to rise above that angle ; a fact which I 
have invariably noticed in such circumstances. 
As if in compensation for this tantalizing illusion, we soon 
afterwards came to a large pool of real water, and had cause for re- 
joicing when we found it perfectly fresh. The newly-discovered 
river, which we had hitherto wronged by the name of the " Brackish 
river," seemed as if kindly resolved to keep us company and lend us 
its friendly assistance during this journey ; for it held the same 
general course, which, according to my calculation, we ought to 
take towards the Colony ; although in its meanderings it often 
quitted us, and as often crossed our track. From this spot, which is 
marked on the map with the name of Freshwater Halt, the river in 
every part upwards affords abundance of wholesome water ; and its 
pools, which occur at very short intervals, are in many places nearly 
confluent. 
Reeds, exactly resembling the common reeds of the English 
rivers, almost every where mark its course. These are a sure 
