258 Capt. H. B. Jackson. On some Phenomena [May 1, 



An examination of these results shows the marked difference between 

 the effects due to the various natures of the intervening land. 



Summarising them for soft rocks, hard limestone, and limestone con- 

 taining a large proportion of iron ores respectively, the percentage of 

 maximum signalling distance through them compared to the open-sea 

 distance is as follows : — 



Soft sandstone, 



shale, &c. Hard limestone. Iron ores. 



Max. distance 81 68 Less than 40 



Min. „ 56 25 „ 23 



Mean ., 72 58 „ 32 



Consider, firstly, the soft rocks : — The two maxima percentages of 

 distance (81 and 80) are over rather low land of no great thickness ; 

 the minimum, 56 per cent., is over high land, half as thick again as in 

 these cases. 



jpoo 



Fig j. 



Distance Jn nautical miles. 





10 



20 30 4o 



da) 



do 



60 

















ah 



A 



r- 2 — 













Represents the positions of the ships at maximum signal distance at sea. 



with intervening 

 band. 



Sandstone. 



Reference as MJJMJih^ Limestone. 



b0 o f f° r u£id n 1 Stated Shale. 



Limestone S; Iron ores. 



Fig. a. 



/poo 



Secondly, the limestone : — The maximum percentage (68) is over the 

 thinnest layer recorded of limestone (3 b), the minimum (less than 

 25) is over a precipitous high mountain through which no signals could 

 be passed at any distance, though they were obtained without difficulty 

 over a low promontory of the same island and of the same forma- 

 tion, when both ships had moved to such positions as to bring the low 

 instead of the high land between them (figs. Sh and Sg). 



