36 
BATHYMETRICAL SURVEY OF 
of these lochs above sea-level is 412 feet, so that the mean height above 
the sea of the entire catchment is about 1347-129 feet. 
The usual practice among engineers is to add 2J per cent, of rainfall 
for each 100 feet of height above rain-gauges. Applying this rule to 
the Loch Vennachar catchment-basin, where we have an observed rain- 
fall of 75-37 inches at an average height of 528 feet, we must add 12-7 
per cent, for the additional 508 feet of mean height, making an 
average annual rainfall over the entire catchment of 84-94 inches. 
This would give an annual fall of rain on the entire catchment equal 
to 14,857,214,000 cubic feet. Applying this rule, in like manner, to 
the Loch Lubnaig catchment-basin, where we have an observed rain- 
fall of 76*25 inches at an average height of 538 feet, we must add 20 
per cent, for the additional 809 feet of mean height, making an 
average annual rainfall over the entire catchment of 91 -5 inches. This 
would give an annual fall of rain on the entire catchment equal to 
15,600,760,000 cubic feet. 
There is another method of estimating the rainfall, without taking 
the mean height of the drainage-area into consideration. Supposing the 
usually accepted increase of 2J per cent, per 100 feet of height, and also 
the mean annual rainfall at the average height of the rain-gauges, to be 
approximately correct, it is possible to calculate the rainfall at any given 
height. For the Loch Vennachar catchment the probable rainfall at 
the same heights and intervals as the contour-lines on the Ordnance 
Survey maps has been calculated from the starting-point of the mean of 
the observing stations 75*37 inches at 528 feet. Thus at the surface of 
Loch Vennachar the rainfall would be about 70*5 inches; at 500 feet 
above the sea, 75*2; at 750 feet, 79*9; at 1000 feet, 84*6 inches; and 
so on, adding 6J per cent, for each succeeding interval of 250 feet. 
Multiplying the area between any two consecutive contour-lines by 
the mean of the two figures calculated for the same two lines should 
give an approximation to the amount of rain falling on that area. The 
result as obtained by this method for the entire catchment-basin flowing 
out of Loch Vennachar is given in the following table: — 
Cubic feet. 
Level of lochs to 500 feet, 
16*53 sq 
uaie miles x 
72*8 inches 
= 2,795,710,000 
500 „ 750 ,, 
10*67 
,, X 
77*5 „ 
= 1,921,117,000 
750,, 1000 „ 
10*35 
., X 
82*2 „ 
= 1,976,514,000 
1000„T250 „ 
9*46 
X 
86*9 ,, 
= 1,909,847,000 
1250 ,,1500 ,, 
10*22 
,, X 
91*6 „ 
=r 2,174,874,000 
1500 ,,1750 ,, 
7*86 
,, X 
96*3 ,, 
= 1,758,476,000 
1750,, 2000 ,, 
5*94 
,, X 
101*0 ,, 
= 1,393,784,000 
•2000,, 2250 ,, 
3*06 
,, X 
105*7 ,, 
= 751,4‘2‘2,000 
2250,, ‘2500 ,, 
0*99 
,, X 
110*4 ,, 
= 253,917,0(X) 
Over 2500 ,, 
0*21 
,7 X 
115*1 ,, 
= 56,154,000 
Total ... 
14,991,815,000 
This result comes very near to that obtained from the calculation 
