12Q FRAGMENTS OF SV1KNCE. 
twisting force just referred to, the promontory under- 
went 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.* 
In the harbor of Gibraltar, on the morning of our 
departure, I resumed a series of observations on the color 
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-aud-twenty bottles were filled with water, 
taken at different places between Oran and Spithead. 
And here let rne 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 un- 
failing courtesy and help. The captain placed at my dis- 
posal his own cockswain, an intelligent fellow named 
Thorogood, who skillfully attached a cord to each bottle, 
weighted it with lead, cast it into the sea, and, after three 
successive rinsings, filled it under my eyes. The contact of 
jugs, buckets, or other vessels was thus avoided; and even 
the necessity of pouring out the water, afterward, through 
the dirty London air. 
The mode of examination applied to these bottles has 
been already described.! 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." Were water of uniform density 
perfectly free from suspended matter, it would, in my 
* No one can rise from the perusal of Mr. Busk's paper without 
a feeling of admiration for the principal discoverer and indefa- 
tigable explorer of the Gibraltar caves, tlie late Captain Frederick 
Brome. 
f " Floating Matter of the Air," Art. " Dust and Disease." 
