1847.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL; 



257 



the refuse, namely, 8,400 can be left on the spot, 8,400 tons of shipping 

 are liberated for oilier purposes of commerce between the colony and the 

 mother country ; and the saving of coal in England, an object not wholly 

 devoid of interest, is immense. 



British Museum. 



Our National Collection may now be compared, not ostentatiously, bat 

 thankfully, with those of other countries ; remembering, also, that our col- 

 lections are little more than half a ceutury old. The ornithological, the 

 conchological, the mammalian departments in the British Sluseum are 

 equal, I believe, to those of any other capital : greatly owing to the talents 

 and labour of the eminent head of that department, Mr. Gray, — whom I 

 see here. The fossil divisions, under the care of my zealous, laborious 

 and able friend, Mr. Kiiaig, are perhaps superior — in some classes, beyond 

 comparison. Last year, there was added to the paheontology of the Mu- 

 seum the unique specimens of the Holitherium of Kaup, the C'ephrtlaspis 

 of Lyell, the Lepidote of Fitton; and the collection of osteology is, as it 

 ought to be, the first in England. The number of visitors, which six jears 

 ago was 319,000, was last year above 700,000 — and the collections of com- 

 parative aniitomy in the Huuleriau Museum are, as they ought to be, the 

 first in the world. 



The following are some of the more interesting papers read in the various 

 Sections of the Association, for which we are indebted to the Atkenaum. 



" Report on Geological Theories of Elevation and Earthquakes." By W. 

 Hopkins. 



This lengthened report embraces too wide a range to admit of our 

 giving at present anydetailed analysis. After having stated certain 

 leading characters of volcanoes, both with reference to the fluid volcanic 

 mass and its containing cavity, the author proceeds to the examination of 

 theories of volcanoes. He regards the chemical theory proposed by Sir H . 

 Davy, and the theory more recently proposed by M. Bischoff, as involving 

 mechanical difficulties of the gravest character. In considering the theory 

 which supposes existing volcanoes to owe their origin to the former fluidity 

 of the earth, the author is led to the discussion of the general theory based 

 on the hypothesis of such fluidity. He examines the evidence afforded in 

 favour of this hypothesis by the accordance between the present ellipticity 

 of the earth, as determined by admeasurement, and its mean density as de- 

 termined hy the experiments of Cavendish and Baily, and the calculated 

 value of their quantities. He then proceeds to consider the mode of the 

 earth's refrigeration and consequent solidification, and the probable extent to 

 which the latter process has already proceeded. Supposing the earth to con- 

 sist of a fluid central nucleus and a solid envelope, it is concluded that the 

 thickness of the latter is probably not less than one-fourth or one-fifth of 

 the earth's radius. This conclusion is drawn from the observed amount of 

 the precession of the earth's pole with that calculated on the hypothesis just 

 stated, respecting the constitution of the earth ; but the author also indi- 

 cated another method by which evidence might be obtained on this point. 

 He showed that if it could be proved by experiment that the temperature of 

 fusion of solid substances is generally increased, even in a small degree, by 

 high pressure, we should have strong reason to believe in the entire solidity 

 of the earth ; and if, on the contrary, it should appear that high pressure 

 has no such effect on the temperature of fusion, we should he led to conclude 

 that the present temperature of the earth is not due to its original heat. He 

 considered such experiments necessary for the further advance of this branch 

 of geology. 



The second part of the report contains a theoretical investigation and 

 examination into the nature and properties of the mechanical effects 

 which would result from the action of such forces. The author proposed to 

 consider the subterranean force as having the nature of an explosion, pro- 

 ducing vibrations over a much wider space than that to which the original 

 force was applied. The vibrations were compared to those produced by 

 striking the end of a solid bar— which are of two kinds. The first set are 

 similar, but infinitely less in extent to vibrations in air. They are produced 

 by compression, and proceed in the direction of the axis of the bar. The 

 second kind are perpendicular to the axis of the bar — Uke the vibrations of a 

 musical chord. In this case the particles of the bar change their form, and 

 the elastic force depends upon their tendency to resume their original shape. 

 The velocity with which vibrations are propagated in the direction of the 

 a>js, is much greater than when their direction is transverse ; and as both 

 usually co-exist, they will after a time separate and become distinct, — both 

 the velocity and the length of the waves of vibration being difterent. If the 

 original impulse, or earthquake shock, is communicated at some distance be- 

 low the surface, the vibrations produced tnay be compared with the disturb- 

 ance produced in water by blowing up a wreck. A. wave will be produced 

 by the alternate compression and dilation of the particles, which will diverge 

 n spheres, equally in all directions, with a constant velocity. In the earth, 

 however, as in the solid bar, there will be two spherical vvaves proceeding 

 outwards with unequal velocities. The apparent motion of these waves, 

 when they reach the surface, will be different, from their real amount 

 of motion below, depending upon the distance of the place of observation 

 from a point immediately over the focus or origin of the force. Assuming 

 the interior of the earth to be homogeneous, and the vibrations produced by 

 earthquakes to be of the kind described, it becomes a leading point to ascer- 

 tain, by observation, the position of the focus from which the vibrations 



onginated. On this subject nothing at present is known. Mr. Hopkins 

 stated, that if self-registering instruments of suflieient dehcacy were placed 

 at two stations in a country subject to earthquakes, the direction of the 

 vibrations would show immediately the point on the surface from which 

 they originated. The depth beneath the surface might also he calculated, 

 from the difference between the apparent movement of the wave ou the sur- 

 face and its real movement in the interior, as given by theory; or it might 

 be ascertained by comparing the relative apparent motion of two waves pro- 

 ceeding with unequal velocities, if means were obtained for recognising the 

 two kinds of waves by instruments indicating the nature of the vibrations. 



Sir H. De la Becue observed, that if the focus of the earthquake were 

 near the surface, the problem would become one of great complexity, on ac- 

 count of the many breaks in the strata, and their difference of composition ; 

 hut if the focus were several hundred miles below the surface, these inequali- 

 ties would be of no consequence. 



Mr. Mallet enumerated the different kinds of waves which do, or may 

 take place, with every earthquake. When the focal point is inlanil, there 

 will be the shock-wave, either single or double; the sound wave in the 

 earth ; and the sound wave in the air, if the original impulse is accompanied 

 with fracture : if the superficial vibration is sufficient, there will also be the 

 sea-wave. When the fucal point is under the sea, as in all great earthquakes, 

 there will be the shock-wave, the sound-wave under the sea, the sound-wave 

 in the air, the great sea-wave, and a smaller, termed the " forced sea-wave ;" 

 if, however, there is no fracture, there will be no sound-waves. It had been 

 ascertained that magnetometers were also " seismometers" of a very delicate 

 kind,— -those at Dublin having indicated from 10 to 20 shocks last year. 



" Report Oil Atmospheric Waves." By W. R. Birt. 

 The author in introducing his fourth report on this subject observed, that 

 in accordance with the resolution adopted at the last Meeting of the Associa- 

 tion, about thirty sets of observations had been obtained from various stations 

 in the British islands ; the extremes of the area embraced being the Orkneys 

 and Jersey in one direction and Galway and Dover in the other. As instances 

 of the increasing interest manifested on this subject, he remarked that he 

 bad been furnished with curves from stations in the north, where the baro- 

 metric movements had been considered to result from the transit of the great 

 November wave. Each of these curves was referred to the same period ; 

 namely, from the 2nd to the 1 7th of November ; and the observers invariably 

 regarded the regular rise and fall that occurred between these epochs as in- 

 dicating a well-marked return of the great symmetrical wave. Mr. Birt, 

 after noticing the remarkable circumstances under which the wave returned 

 last antumn — so remarliable that they had no small tendency to mark the 

 wave in the south-eastern part of the island — stated that the projected curve 

 at London strikingly developed its essential features ; the Jive subordinate 

 waves were well seen, although the inflexions were not strong, owing to the 

 small altitude of the wave on its last return, scarcely exceeding half an inch 

 — its whole development occurring above thirty inches prevented the bold- 

 ness of the inflexions particularly noticed on the occasion of its return itt 

 1812. The author then proceeded to notice the essential features of the 

 curves as obtained from observations at Ramsgate, St. Vigean's near Arbroath, 

 east coast of Scotland, the Orkneys and Western Isles, Applegarth Manse, 

 Dumfries-shire, Largs, Limerick, Galway, Helstone in Cornwall, and St. 

 Heller's, Jersey. Cur limits will not permit us to give in detail the resem- 

 blances and differences of these curves, exhibiting, as they do, the distribu- 

 tion of pressure around Great Britain and Ireland, which the author traced 

 from the south-eastern point towards the north-west ; but the report will be 

 printed in the forthcoming volume of the Transactions. M'e may, however, 

 here notice that attention was called to the principle which the author laid 

 down in his report of last year, " that the barometric curve, including a com- 

 plete rise and fall at any one station, does not represent any reality in nature, 

 but is the effect of two or more systems of waves or currents moving in dif- 

 ferent directions and crossing each other at various angles." He also pointed 

 out the great extent of oscillation (nearly double) observed in the north- 

 west as compared with the south-easterly observations. The great wave 

 commenced on the 2nd of November ; at the northern stations it culminated 

 on the r2th ; at the south-eastern on the 9th ; and it terminated on the 17th. 

 In explaining the differences of epoch as indicating the transit of the crest 

 being much earlier in the south-east than in the north, Mr. Birt remarked 

 that the observations clearly showed that the barometer passed two maxima, 

 one on the 9th, the other on the 12th; and that the whole extent of the 

 British isles might he divided into ttm barometric areas, distinguished in one 

 case by the superiority of the maximum of the 9tb, and in the other by the 

 superiority of the maximum of the 12th. A line passing between .Arbroath 

 and Newcastle, south of Dumfries, and between Ireland and Wales, separates 

 these areas. North-west of this line we find the maximum of the 12th supe- 

 rior : south-east of it we find the maximum of the 9th superior. The maxi- 

 mum of the 9th Mr. Birt regarded as the central wave forming the crest of 

 the great wave, and the maximum of the 12th he considered as the crest of 

 the first subordinate wave on the posterior slope. The author next proceeded 

 to examine the distribution of pressure as manifested by these observations; 

 from which, in connexion with the features of the projected curve, he de- 

 duced the following results : — 1st. The return of the great symmetrical wave. 

 This occurred in the south-eastern angle of our island under very peculiar 

 circumstances. The area of greatest symmetry is closely in accordance with 

 the results of former discussions, and goes far to confirm the result deduced 



33 



