of Edinburgh, Session 1871 - 72 . 555 
even 105 fathoms, — the width there being barely three-fourths of a 
mile. 
The surface water over these great depths is of remarkable 
purity. Its saline matter is very scanty, and the colouring organic 
matter equally so. Still it has a faint yellowish colour. On Sep- 
tember 21st, the second day after heavy rain, incessant for twelve 
hours, a white porcelain basin, 4 inches in diameter, disappeared in 
18 feet of water; on 11th October, after many days of alternate 
rain and drought, in 15 feet; and on 18th November, after four 
days of dry weather, in 14 feet, but in feeble sunshine.* After 
long drought there is little doubt that the colour would be less, for 
it will be seen subsequently, that as the streams pour in fresh sup- 
plies of water, there is reason to suppose that these penetrate little 
before they run off, and consequently the coloured flood waterfrom the 
streams will colour for some time the superficial waters of the lake. 
On 18th November, the water taken from the surface of Loch 
* This is a good method of ascertaining the relative colour of waters if it be 
smployed with due precautions. The trial should be made in sunshine — 
when the sheet of water is quite calm— between 9 a.m. and 3 p.m., so that 
the sun’s rays may not fall too obliquely on the water, and with the back to 
the sun, and, best of all, on the shady side of a boat. If all these conditions 
be reversed, vision will penetrate scarcely half so deep as when they are all 
observed. In my recent trials I have not found a white object visible at a 
greater depth than 21 feet, viz., on Loch Lomond on the 6th May. But, 
from observations made many years ago, I am satisfied that, after long dry 
weather, some river waters will allow such an object as a white porcelain 
basin to be seen at a much greater depth, with due attention to the condi- 
tions now mentioned. Having a recollection of seeing it stated long ago, 
that the water of the Lake of Geneva was so clear, that objects could be dis- 
tinguished in it at a very great depth, I applied to Dr Coindet of Geneva for 
precise informatien, for which he referred me to Professor Forel of Lausanne. 
To Professor Forel’s kindness I am indebted for the following interesting 
facts : — In the spring of 1869, using a white-painted sheet of iron, 15 inches 
by 12, he found that the utmost depth at which it could be seen was 13 
metres, or 44 feet. The transparency is much affected by locality, and very 
much too by season. In winter and spring it is greatest, in summer and 
autumn least. In the Bay of Morges, objects may be seen distinctly at the 
bottom in winter at a depth from 13^ to 20 feet, while in summer they are 
barely visible through 7 feet. This difference is greatest near the shore, at 
the bottom of bays, and near villages or towns. It is least around promon- 
tories, far from land, and at a distance from human habitations. In autumn 
the change from obscurity to transparency usually takes place early in October, 
and is completed in three days ; in summer, the reverse change takes place 
