HARDWICKE'S SCIENCE-GOSSIP. 



247 



owing to tlie great infertility of the eggs, and the 

 shortness of the clutchfes, 10 per cent, only contain- 

 ing live eggs, against 20 per cent, in the previous 

 year. But as the season advanced, matters greatly 

 improved, bringing the season up to that of 1889, 

 when the average brood was three young birds. In 

 my report of last year I mentioned that very many 

 young birds are turned out of the nests either dead or 

 to die, and that so many more of these little outcasts 

 were seen towards the close of the season than in 

 the early part, is fair presumptive evidence that 

 the fertility of the eggs improved as the season 

 advanced. 



The following figures show tiie slight variation in 

 the brood during the past six seasons : — 



1886, average brood, 3'j young birds 



1807 ,, ,, 3s >> ji 



1888 



18S9 „ 



1890 



1S91 



-1I 

 >> o j 



)' 3 



There can be no doubt that the stock of sparrows 

 at the present time is below the average, owing to 

 the severity of the late winter, and the objectionable 

 practice of placing a price upon their heads. 



Joseph P. Nunn. 



Royston. 



THE LIZARD ROCKS AND THE 

 MARINER'S COMPASS. 



THE favourite problem of Mr. Thomas Clark, of 

 Truro, and ]Mr. Howard Fox, F.G.S., of 

 Falmouth, on the magnetic rocks of the Lizard and 

 district, was ably ventilated at the recent annual 

 exhibition of the Royal Cornwall Polytechnic Society. 

 Both these gentlemen have been laborious wcirkers on 

 the subject, and, as a consequence, their remarks carry 

 weight. But while on the one hand Mr. Clark 

 considers he has proved, by carefully conducted 

 experiments, the magnetic properties of some of the 

 Lizard Rocks, Mr. Fox, cautious scientist that he is, 

 is inclined to doubt that these rocks have any 

 appreciable effect on the mariner's compass at a 

 distance of two hundred yards. At the Polytechnic 

 meeting r^Ir. Clark demonstrated that the most highly 

 magnetic rocks were those in the neighbourhood of 

 Coverack and Black Head, The Manacles Rocks, 

 stretching out into the entrance to Falmouth Bay, 

 exerted very little influence on a magnet suspended 

 by a silk thread. Mr. Fox also proved to the 

 audience, by a series of skilful experiments on 

 specimens of the gneiss and porphyritic diorites 

 which compose the outer Lizard Rocks, that they 

 were only magnetic to a small degree. Both gentle- 

 men having conclusively demonstrated the more or 

 less magnetic influence of the rocks forming the coast- 



line from the Lizard to the Manacles, the question 

 turned on the effects of these rocks on the mariner's 

 compass. Mr. Fox read some interesting extracts 

 from the philosophical transactions of the Royal 

 Society of 1890, which showed that Messrs. Riicker 

 and Thorpe, after five years' close study of the 

 question, seemed confident that the effect of highly 

 magnetic rocks was practically inappreciable at a 

 distance of two hundred yards. Mr. Fox said the 

 influence of the serpentine forming the Black Head 

 could be easily tested by sailing close to the point in 

 and out of Kennack Bay, and steering by marks on 

 shore independent of the compass. The magnetic 

 rocks exhibited by Mr. Clark, he contended, had 

 much less influence on a mariner's compass than on 

 a delicately suspended bar magnet, and he considered 

 that, though Mr. Clark had proved that many of the 

 rocks in the Lizard district were strongly magnetic, 

 until fresh evidence was forthcoming they must take 

 Riicker's and Thorpe's investigations to prove that 

 magnetic rocks had no appreciable effect on the 

 mariner's compass at a distance of two hundred yards. 

 The report of these two gentlemen on the magnetic 

 survey of the United Kingdom extended over two 

 hundred and seventy-five pages of the quarto volume, 

 and was now the standard authority. They found 

 that the most highly magnetic rock was the basalt in 

 the Island of Canna, in the Hebrides, and concerning 

 this they have written: "Although a compass will, 

 within a foot or two of the basaltic columns around 

 the summit of Compass Hill, in the Island of Canna, 

 in the Hebrides, so disturb the needle as to cause a 

 deflection of over 25°, this disturbing influence 

 diminishes very rapidly at a distance. At an hori- 

 zontal distance of eighty yards from the eastern side 

 of this hill the disturbance diminishes from 33° from 

 the top of the hill to i • 6° from its base." On 

 leaving Canna in 1884, they sailed as close as possible 

 to the north of the island, and frequently took 

 compass-bearings and poiats of Skye, but were 

 unable to detect the smallest defleciions of the needle. 

 In 1 888 they approached the island in a yacht from 

 the north, and when about three miles distant the 

 yacht was directed to a mark on Rum, by which its 

 course could be kept without reference to the 

 compass. They passed Compass Hill within two 

 hundred yards without observing any deflection of 

 the compass, and they were quite certain that if there 

 was any, it was less than i '5°. 



In the discussion, Mr. Henderson, C.E., mentioned 

 that in Botallack mine, situated near Cape Cornwall, 

 it was a most difficult matter to secure a bearing, 

 owing to the magnetic rocks, and he had had to go 

 down to one hundred and forty-five fathoms before he 

 could get a bearing. The debate was the most 

 instructive one Cornwall has been favoured with on 

 this subject. At the close Mr. J. H. Teall, F.R.S., 

 expressed a hope that Mr. Clark would continue his 

 investigations and give them greater quantitative 



