November 1, 1888.] 



♦ KNOVVTLEDGE ♦ 



13 



becomes an efBcient lightning - conductor. For similar 

 reasons, it is dangerous to seek the shelter of a lofty build- 

 ing (not protected by a lightning-conductor) in a thunder- 

 storm. One of the most terrible catastrophes known in the 

 history of thunderstorms occurred to a crowd of persons 

 who stood in the porch of a village church waiting till a 

 thunder-shower should have passed away. 



In the open air, when a heavy thunderstorm is progress- 

 ing, and no shelter near, the best course is to place one's 

 self at a moderate distance from some tall trees. Franklin 

 considered a distance of about 15 or 20 feet the best. 

 Henley also considered 5 or 6 yards a suitable distance in 

 the case of a single tree. But when the tree is lofty, a 

 somewhat greater distance is preferable. 



The reader need hardly be reminded, perhaps, that the 

 necessity for taking these precautions only exists when the 

 storm is really raging close at hand. When the interval 

 which elapses between the lightning-flash and the thunder- 

 peal is such as to show that the storm is in reality many 

 miles away, it is altogether unnecessary to take precautions 

 of any sort, however brilliant the flash may be, or however 

 loud the peal. It must be noticed, however, that a storm 

 often travels very rapidly. If the interval of time between 

 the lightning and the thunder is observed to diminish 

 markedly, so that the storm is found to be rapidly approach- 

 ing the observer's station, the same precautions should at 

 once be taken as though the storm were raging immediately 

 around him. So soon as the interval begins to grow longer, 

 it may be inferred that the storm has passed its point of 

 nearest ajiproach, and is receding. But the laws according 

 to which thunderstorms travel are as yet very little under- 

 stood ; and it Ls unsafe to assume that because the interval 

 between flash and peal has begun to increase after having 

 diminished, the storm is therefore wrtainlij passing away. It 

 must be in the experience of all who have noted the circum- 

 stances of thunderstorms, that when a storm is in the 

 neighbourhood of the observer, the interval between the 

 fla.sh and the thunder-peal will often increase and diminish 

 alternately several times in succession. It is only when the 

 interval has become considerable, that the danger may be 

 assumed to have passed away. 



GEOLOGY IN LONDON. 



By W. Jerome Hariuso.\, F.G.S. 



A VING described the various museums and 

 collections of specimens which are likely to 

 assist the student of geology in London, we 

 may now proceed to consider where he may 

 obtain the active teaching which shall give 

 life to the fossils, and enable the stones to 

 pre.ach their own sermons. First, however, 

 we may mention that since the appearance of our former 

 ai-ticle the extensive collections formed by the late Henry 

 Christy to illustrate the life of prehistoric man, by com- 

 paring his weapons, tools, etc. with those of modern savage 

 tribes, have been arranged in the galleries of the British 

 Museum, Great Russell Street, and are now accessible to 

 the public. 



There is another point connected witli the first part of 

 our subject to which we may perhaps allude here. The 

 streets and buildings of London are in tlicmsolves a geo- 

 logical museum to those who cou.sider them with an observ- 

 ing eye. The ]>avoments are largely formed of flat slabs of 

 sandstone from the carboniferous rocks of the West Riding 



of Yorkshire — York stone, as the builders call it. Where 

 the horse road is paved with " setts " (cutes of stone 

 about six inches along each edge) these can be seen 

 — after a good shower of rain — to be either a compact 

 black stone, in which case they are a basalt (probably 

 from the Rowley Hills, near Dudley), or a speckled stone 

 formed of crystals of felspar, quartz, mica, i&c, consti- 

 tuting a granite or a syenite. The hills of Charnwood 

 Forest, in Leicestershire, yield enormous quantities of stone 

 for this purpose ; in the chief quarry at Mountsorrel, which 

 is veiy visible from the Midland Railway between Leicester 

 and Loughborough, about 700 men find regular employment. 

 The Channel Islands — especially Guernsey — and Cornwall 

 also yield excellent granite, mostly of a whitish colour, but 

 as the stone from these localities can be obtained in large 

 blocks free from joints, it is more valuable for building pur- 

 poses. The Thames embankment is chiefly of Guernsey 

 granite. As to the materials of which London houses are 

 built, brick is of course the cheapest, because the necessary clay 

 is furnished by the " London Clay," a bed of which, sevei'al 

 hundred feet in thickness, encircles the metropolis. The red 

 colour of a common brick is due to a small percentage of per- 

 oxide of iron. Limestone — white or yellowish-white in colour 

 — is largely supplied from the quarries of oolitic limestone 

 round Bath in Somersetshii'e and Portland in Dorsetshire. 

 Westminster Hall and the Houses of Parliament are built of 

 magnesian limestone {J'cnnian) from Nottinghamshire. It is 

 Cixsy to distinguish a lime.stoue by the way in which it 

 effervesces when an acid — vinegar, for example, though 

 dilute hydrochloric acid is preferable — is poured upon it. 

 The effervescence is due to the escape of carbonic acid gas. 

 The sandstones, also largely employed for public build- 

 ings, come mainly from the millstone grit {Oarhoniferous 

 series) of the north and west of England. The roofing-slates 

 come chiefly from the great quarries in the Cambnan Rocks 

 round Snowdon ; although the quarries at Swithland and 

 Groby in Charnwood Forest (Leicestershire) have of late 

 years yielded a limited supply. The St. Paucras Hotel is 

 roofed with Charnwood slates. 



INSTITUTIONS WHERE GEOLOGY IS TAUGHT. 



Until quite lately the Jermyn Street Geological Museum 

 had associated with it a Government Institution known as 

 the "Royal School of Mines," first established in 1851. 

 But lack of accommodation caused a gradual transfer of the 

 students to the " Science Schools " at South Kensington — a 

 large red brick and terra-cotta building in Exhibition Road, 

 facing the Exhibition entrance. The change was completed 

 in 1882, and the Institution is now known as " The Normal 

 College of Science and Royal School of Mines." The teach- 

 ing of "mining" proper is, however, still continued at 

 Jermyn Street, by Professor Warington W. Smyth, since 

 the splendid collections of models, ores, <fec., there accumu- 

 lated offer special advantages. 



At South Kensington the geological classrooms are at the 

 top of the building, and in the work here Professor J. W. 

 Judd is assisted by Mr. Cole as demonstrator, and I\Ir. 

 Rutloy as lecturer on mineralogy. Professor Judd's name is 

 well known to every geologist. His work on the Geological 

 Survey was excellently done, and his travels on the conti- 

 nent enabled him to write a book on " Volciuioes," which 

 at once beaime an authority. He is now President of the 

 Geological Society. For those who desire to take the Asso- 

 ciateshij) of the School of Mines a three years' course is 

 provided at South Kensington, and there is no better train- 

 ing in science given anywhere ; but occasional students are 

 also admitted to the geological class ou pjiyment of U. for 

 the lectures, and 8/. for the laboratory work for the session. 



