38 BRITISH CENTRAL AFRICA 
a deep valley with abrupt cliffs ; though this condition of course restricts its 
area of growth. 
Still, although we must, I think, admit a certain diminution in rainfall owing 
to the decrease of forest or other causes, the rate at which this decrease is going 
on has been exaggerated, and as we come to know the country better and our 
records grow with years of occupation, we see that there are signs of cycles of 
greater and less rain dependent on atmospheric conditions which we have not yet 
realised. The marks on the rocks show that during some ages there has been a 
slight—but a very slight—fall in Lake Nyasa, varied by periods of extraordinary 
diminution as for instance some seventy years ago when according to the natives’ 
traditions the north end of the lake became so shallow between Deep Bay 
and Amelia Bay that a chief and his men waded across where it is now many 
fathoms deep. The highest watermark on these polished rocks is perhaps at 
most six feet above the present high levels of the lake in good rainy seasons. 
In years of relative drought Lake Nyasa may be as much as six feet below its 
best rainy season average. This means, of course, that instead of there being 
nine feet of water on the bar of the Shire where that river quits the lake there 
are only three feet ; consequently the navigability of the Shire in the dry 
season becomes much embarrassed and in these bad years it can only be 
navigated all the year round 
by vessels not drawing more 
than one and a half feet. 
Yet we know that in the later 
“ fifties ” and early “ sixties ” 
Livingstone constantly travelled 
up and down the Shire on a 
vessel drawing five feet. Even 
in the year 1889 the James 
Stevenson which draws about 
three feet of water was able to 
navigate the Shire through al¬ 
most all the year up to the 
Murchison falls, while vessels 
of five feet draught have in like 
manner navigated the Upper 
Shire above the falls. But from 
1891 till 1896 the Shire fell 
lower and lower until at last 
not even Chiromo was the limit 
of navigation from the sea, 
but the Linda rapids near the 
Zambezi, while the Upper Shire 
was practically divided into a 
few navigable stretches with 
very shallow water in between. 
But after the rainy season of 
1895-96 Lake Nyasa rose to a 
height which had not been 
reached for many years and is apparently still continuing to rise. The result is 
that the Lower Shire is now as navigable as it was in Livingstone’s day, while 
on the Upper Shire many of our low-lying stations are threatened by the flood 
