orwich, and
afterwards printed in the 'Transactions' of the Congress. [Footnote:
In this essay Mr. Busk refers to the previous labours of Mr. Smith, of
Jordan Hill, to whom we owe most of our knowledge of the geology of
the rock.] Long subsequent to the operation of the twisting force
just referred to, the promontory underwent various changes of level.
There are sea-terraces and layers of shell-breccia along its flanks,
and numerous caves which, unlike the inland ones, are the product of
marine erosion. The Ape's Hill, on the African side of the strait,
Mr. Busk informs me has undergone similar disturbances. [Footnote: No
one can rise from the perusal of Mr. Busk's paper without a feeling of
admiration for the principal discoverer and indefatigable explorer of
the Gibraltar caves, the late Captain Frederick Brome.]

*****

In the harbour of Gibraltar, on the morning of our departure, I
resumed a series of observations on the colour of the sea. On the way
out a number of specimens had been collected, with a view to
subsequent examination. But the bottles were claret bottles, of
doubtful purity. At Gibraltar, therefore, I purchased fifteen white
glass bottles, with ground glass stoppers, and at Cadiz, thanks to the
friendly guidance of Mr. Cameron, I secured a dozen more. These
seven-and-twenty bottles were filled with water, taken at different
places between Oran and Spithead.

And here let me express my warmest acknowledgments to Captain
Henderson, the commander of H.M.S. "Urgent," who aided me in my
observations in every possible way. Indeed, my thanks are due to all
the officers for their unfailing courtesy and help. The captain
placed at my disposal his own coxswain, an intelligent fellow named
Thorogood, who skilfully attached a cord to each bottle, weighted it
with lead, cast it into the sea, and, after three successive rinsings,
filled it under my own eyes. The contact of jugs, buckets, or other
vessels was thus avoided; and even the necessity of pouring out the
water, afterwards, through the dirty London air.

The mode of examination applied to these bottles has been already
described. [Footnote: On Dust and Disease, p. 168.] The liquid is
illuminated by a powerfully condensed beam, its condition being
revealed through the light scattered by its suspended particles. 'Care
is taken to defend the eye from the access of all other light, and,
thus defended, it becomes an organ of inconceivable delicacy