TRANSACTIONS OF THE SECTIONS. 6 



And the United-States Naval Observatory, full of the very highest science, under 

 the command of Admiral Davis! If, to get on to Precession and Nutation, I had 

 resolvt'd to omit telling you that I had there, in an instrument for measuring pho- 

 togi-aphs of the Transit of Venus shown nie by Professor Ilarkness (a young Scots- 

 man attracted into the United-States Naval Service), seen, for the first time in an 

 astronomical observatory, a geometrical slide, the verdict on the disaster on board 

 the 'Thunderer,' published while I am writing this address, forbids me to keep any 

 such resolution, and compels me to put the question — Is there in the British Navy, 

 or in a British steamer, or in a British land-boiler another safety-valve so con- 

 structed that by any possibility, at any temperatm'e or under any stress, it can jam ? 

 and to say that if there is, it must be instantly con-ected or removed. 



I ought to speak to you, too, of the already venerable Harvard University, the 

 Cambridge of America, and of the Technological Institute of Boston, created by 

 William Rogers, brother of my late colleague in this University (Glasgow), Henry 

 Rogers, and of the Johns Hopkins University of Baltimore, which with its youthful 

 vigour has torn Sylvester from us, has utilized the genius and working-power of 

 Roland for experimental research, and three days after my arrival in America sent 

 for the young Porter Poinier to make him a Fellow ; but he was on his deathbed, 

 in New York, "begging his physicians to keep him alive just to finish his book, 

 and then he would be willing to go." Of his book, 'Thermodynamics,' we may 

 hope to see at least a part, for nuich of the manuscript and good and able 

 friends to edit it are left; but the appointment to a Fellowship in the Johns 

 Hopkins University came a day too late to gratify his noble ambition. 



But the stimulus of intercourse with American scienfillc men left no place in 

 my mind for framing or attempting to frame a report on American science. Dis- 

 turbed by Newcomb's suspicions of the eirth's irregularities as a time-keeper, I 

 could think of nothing but precession and nutation, and tides and monsoons, and 

 settlements of the equitorial regions, and meltings of the polar ice. Week after 

 week passed before I could put down two words which I could read to you here 

 today ; and so I have nothing to ofler you for my Address but 



Review of Evidence rer/ardinr/ the Ph)/sicfd Condition of the Earth: its internal 

 Temperatii,re ; the Fluidity or SoUdit}/ of its interior Substance ; the 

 Eic/idity, Elasticity, Plasticity, of its Eocterncd Figure ; and the Per- 

 manence or Variability of its Period and Axis of Rotation. 



The evidence of a high internal temperature is too well known to need any quo- 

 tation of parliculars at present. Suthce it to say that below the uppermost ten 

 metres stratum of rock or soil sensibly affected by diurnal and annual variations of 

 temperature there is generally found a gradual increase of temperature downwards, 

 approxinrating roughly in ordinary localities to an average rate of 1° Centigrade 

 per thirty metres of descent, but much greater in the neighbourhood of active vol- 

 canoes and certain other special localities, of comparatively small area, where hot 

 springs and perhaps also sulphurous vapours prove an intimate relationship to vol- 

 canic quality. It is worthy of remark in passing that, so far as we know at present, 

 there are no localities of exceptionally small rate of augmentation of imderground 

 temperature, and none where temperature diminishes at any time through any con- 

 siderable depth downwards below the stratum sensibly influenced by summer heat 

 and winter cold. Any considerable area of the earth of, say, not less than a kilo- 

 metre in anv horizontal diameter, which for several thousand years had been covered 

 by snow or ice, and from which the ice had melted away and left an average surface 

 temperature of 13° Cent., would, during 9(J0 years, show a decreasing temperature 

 for some depth down from the surface ; and 3600 years after the clearing away of 

 the ice would .still show residual effect of the ancient cold, in a half rate of aug- 

 mentation of temperature downwards in the upper strata, gradually increasing to 

 the whole normal rate, which would be sensibly reached at a depth of 000 metres. 



By a simple effort of geological calculus it has been estimated that 1° j er -30 

 metres gives 1000° per .30,000 metres, and 333.3° per 100 kilometres. This aiith- 

 metical result is irrefragable ; but what of the physical conclusion drawn from it 



