!26 FRA OMENTS OF SCIENCE. 



twisting force just referred to, the promontory under- 

 went various changes of level. There are sea terraces 

 ami 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 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 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. f 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, the late Captain Frederick 

 Brome. 



f " Floating Matter of the Air," Art. " Dust and Disease." 



