58 TRAVELS IN 
new hill, or that the only one which stood in that direction, 
at the distance of a long day's journey, must have greatly in- 
creased of late in its dimensions. Being directed to turn 
his eyes from time to time towards the quarter on which it 
stood, he perceived, with amazement, that, as the day ad- 
vanced, the hill gradually sunk towards the horizon, and at 
length totally disappeared. The eiTors of sight, occasioned 
by the refractive power of the air, are so singular, and some- 
times so very extraordinary, as hitherto to have precluded the 
application of any general theorem for their correction, as it 
is not yet ascertained even through what medium rays of 
light, in their passage, suffer the greatest and least degree of 
refraction. Were this precisely known, observations on the 
subject might lead to a more intimate knowledge of the nature 
of the different currents of air that float in the atmosphere, 
and which without doubt are the cause of extraordinary ap' 
pearances of objects viewed through them. A gentleman, 
(the late Mr. Ramsden,) to whom the world is much indebted 
for his many ingenious and useful inventions and discoveries, 
once proposed to determine the refractive power of different 
liquids and aeriform fluids ; but he died before he had com- 
pleted a course of experinjents on this subject, which is not 
less important than curious. 
Our cattle being well refreshed on the meadows of De Beer 
Valley, we advanced about twenty miles, and encamped for 
the night on the banks of Hottentot's river, in the narrow deep 
channel of which were only a few stagnant pools of muddy 
water. Here we were met by some of the inhabitants of Cam- 
deboOj who, being apprised of the approach of the landrost. 
