396 



NATURE 



[Sept. 9, 1875 



Although no one acquainted with the care bestowed upon this 

 description of work at Greenwich would for one moment think 

 of impugning the accuracy of these estimations, they show pre- 

 cisely the excess of whole seconds that is taken in the before- 

 naentioned article as indisputably proving the carelessness of the 

 tabulations at the Kew Observatory. 



As regards these averages, it is to be remarked that with one 

 slight exception all the numbers that are above or below the 

 theoretical average in one example are above or below in all, and 

 that there is only one case in which the range of difference 

 exceeds 3 per cent. The partiality shown for the figures o and 

 4 is also most marked, and of itself would be enough to show 

 that the same person had made all the estimations. 



There is another light in which We may regard these results, 

 which still more plainly indicates my meaning. The decimals 

 •I, '2, &c., ought to include all possible positions of the puncture 

 between '05 and "15, between '15 and "25, and so on; but 

 according to the reader of the chronographic sheets, 'i includes 

 only those positions of the puncture between "081 and '151 ; "z 

 includes those between TSi and "230; -3 those between '230 and 

 •319; '4 those between -319 and '481, and so on. Thus the 

 error of any single determination is very small indeed, a remark 

 that will apply equally to the tabulations 

 Meteorological Office. 



To show that different observers have very different idiosyn- 

 crasies, I may append the following averages similarly deter- 

 mined, this time from the purely astronomical estimations of the 

 time of transit of stars across the well-defined spider lines of 

 the telescope by the method known as eye and ear observa- 

 tion, these estimations being made on a precisely similar principle. 

 From the Greenwich observations of 1864 I find 206 such esti- 

 mations bv Mr. Dunkin, the standard observer at that time ; 259 

 by Mr. Ellis ; and lastly, 500 by myself in-the present year, made 

 at this observatory, yield the following : — 



Although founded on rather too few estimations, there is little 

 doubt that the salient features would be preserved in a more 

 extended discussion. Thus D's avoidance of whole seconds and 

 the adjacent numbers- i and 9, E's avoidance of the former of 

 these, and my own of the latter, may be expected confiJently, 

 however large a number of estimations are taken into account. 

 The universal fondness for 8 is also noteworthy. 



Orwell Park Observatory, John J. Plummer 



near Ipswich 



Source of Volcanic Energy 



In your report of the meeting of the Geological Society in 

 Nature, vol. xii. p. 79, I find notes of a communication sub- 

 mitted by the Rev. O. Fisher, F.G.S., on Mr. Mallet's theory 

 of volcanic energy, and as I consider Mr. Mallet's paper to be 

 one of surpassing value, I wish to make a few remarks on the 

 criticism of it by Mr. Fisher. Mr. Fisher objects to the possi- 

 bility of assuming high local temperatures to be produced by the 

 transformation of tangential forces into heat, within the earth's 

 erust. 



If the strata of which the earth's crust is composed could be 

 represented in a diagram by so many concentric circles of perfect 

 regularity, the crushing force resulting from tangential pressures 

 caused by the regular contraction of the mass would of course be 

 equal all through the mass ; but, as a matter of fact, such a dia- 

 gram would not be a faithful representation of the lie of strata 

 in the earth's crust. These strata occur at all sorts of angles, 

 and are broken in upon by faults of great extent ; so the pressures 

 produced upon various parts of the earth's crust are far from 

 equal. These inequalities are also increased by the differences 

 in density of strata as also by the thinning out of strata of the 

 same density. 



For instance, a strain may occur somewhat in the manner of 

 the annexed diagram. A set of strata may bear upon a point A, 

 considering the forces to act in the direction BA, CA, and so 

 cause the pressure upon a square foot at A to be a hundredfold 

 greater than on a square foot at E. The work done, therefore, 

 may not be equally distributed over certain areas : but forces 



may converge upon various points, and if the work is thus inten- 

 sified in certain points, the heat developed in such points must 

 be greater than where the forces are not so concentrated. It 

 seems to me, then, that the rocks at A may be crushed to fusinq;- 

 point by converging forces, while at the same time the rocks of 

 the same set of strata at B may be at a much lower temperature. 



If what I have attempted to point out contains no " untenable 

 assumption," the possibility of the developed heat being local- 

 ised remains intact ; and this is certainly the main feature of Mr. 

 Mallet's theory. 



Mr. Fisher's objection to the primeval formation of our present 

 existing ocean beds and continents seems a fair one, notwith- 

 standing the fact of the remarkable steepness of the western coasts 

 of all continents remarked upon by Mr. Mallet, but this remarkable 

 similarity of formation may be no more remarkable than the 

 fact of all the great promontories of the world pointing to the 

 south and none to the north. Still, however, Mr. Mallet's paper 

 may help us, for if the tangential pressures produced in the 

 earth's crust be sufficient in some cases to produce long lines ot 

 volcanic activity, may they not in other cases be resolved into 

 motions acting in various directions and causing the upheaval of 

 continents and depression of ocean beds ? 



In conclusion I may remark that if mere cooling is not con- 

 sidered sufficient fo account for the development of such forces, 

 may not forces produced by gravitation acting in the very same 

 direction be well acknowledged ? Not mere gravitation of the 

 surface upon a retreating nucleus, which of course is part of 

 Mr. Mallet's theory, but gravitation of the whole mass to itself, 

 which enormous source of energy must also express itself in tan- 

 gential pressures in the more resisting crust of the earth ? 



Kenmare W. S. Green 



Sanitary State of Bristol and Portsmouth 



In reference to the peculiar low mortality of some large towns 

 in Great Britain, stated in the abstract of a communication 

 to the Scottish Meteorological Society in Nature, vol. xii. 

 p. 281, as Portsmouth and Bristol, in contradistinction to others 

 apparently in similar circumstances, having a high death-rate, I 

 beg leave to point out that each of these towns is differentiated 

 from the others mentioned in the paper in a social point of view 

 more than in physical conditions. There is a large district in 

 each of them, inhabited chiefly by visitors, tourists, retired pro- 

 fessionals, and mercantile people, who take up their quarters in 

 Southsea and Clifton, for the period of the regular seasons in 

 each, or for limited tenure of occupation, either with reference to 

 health, pleasure, or education of their families. 



These divisions or quarters of Portsmouth and Bristol are 

 under different physical conditions from the parent cities they 

 are attached to, in that they are of separate growth, of later 

 date of construction, better built, and inhabited by a wealthier 

 class of people. 



They might be compared to the apple-grafting on a crab-tree, 

 on the old stem of which they flourish, but bear more showy 

 flowers and more luxuriant fruit, and they thus tend to ameliorate 

 the inherent deficiencies of the original tree by adding a higher 

 and more cultivated life. 



Topographically speaking, again, these two districts are entirely 

 different from eacti other, though equally healthy, as above 

 stated, Southsea being built upon a plain near the sea, and 

 Clifton being built upon a hill above a river : the one lies on 

 gravel and the other on limestone, so that these and other 

 material circumstances, oddly enough, can scarcely be thought 

 likely to produce a common result on their sanitary state. 



The original towns of Portsmouth and Bristol, however, are 

 nearly alike in some points, but not in others, Both are shipping 

 ports, both are on tidal harbours, both are built along the banks 

 on each side, and are therefore low in altitude above the sea ; 

 but the former lies on gravel, while the latter is built on alluvium 

 and red sandstone. Most other large towns are of a homo- 

 geneous constitution, as Manchester in manufactures, Liverpool 

 in shipping, Scarborough as a seaside resort, and Cheltenham 

 as an inland watering-place; but Portsmouth and Bristol are 

 peculiar in having this coublc social compos'tion of a shipping 



