NATURAL PHILOSOPHY". 193 



perpend icular to gravity. If a kind of mean surface be conceived intersect- 

 ing this, so as to leave equal volumes above of elevations, and of depres- 

 sions below it, it is not allowable to assume that such a surface is perpendic- 

 ular to gravity. The mean surface of the solid crust of the earth would not 

 be perpendicular to gravity, if, after the process of solidification had com- 

 menced, any extensive changes in the distribution of matter in the earth's 

 interior could take place. If the fluid matter in solidifying underwent no 

 change of volume, the forms of the strata of equal density within the earth 

 would be the same at every stage of its solidification. But if, as observa- 

 tion indicates, such fused matter, on passing to the solid crystalline state, 

 should diminish in volume, the pressure on the remaining strata of the fluid 

 would be relieved, and they would tend to assume a greater ellipticity than 

 they had when existing under a greater pressure. The general result of this 

 action would manifestly be to produce a change in the direction of the at- 

 tractive forces at the outer surface of the solid crust. The direction of a 

 plumb-line would be slightly altered so as to slightly increase the apparent 

 latitudes of places over a zone intermediate between the equator and poles. 

 M. D'Abbadie stated several cases which he had met with, where monu- 

 ments existed which showed that the direction of gravity, at some former 

 period, must have been very different in relation to those particular portions 

 of the earth from what it now was. Other Members also noticed deviations 

 of the plumb-line from its normal position, and some of them which seemed 

 to depend on the season of the year. The President, Dr. Robinson, stated 

 that he was the first to direct attention to those changes of level which de- 

 pended on the season of the year. This he was led to observe from the fact 

 that the entire mass of rock and hill on which the Armagh Observatory was 

 erected was found to be slightly, but to an astronomer quite perceptibly, tilted 

 or canted at one season to the east, at another, to the west. This he had at 

 first attributed to the varying power of the sun's radiation to heat and ex- 

 pand the rock throughout the year ; but he since had reason to attribute it 

 rather to the infiltration of water to the parts where the clay, slate, and 

 limestone rock met in their geological arrangement. The varying quantity 

 of this through the year he now believed exercised a powerful hydrostatic 

 energia, by which the position of the rock was slightly varied. 



OX THE EECEXT DISCOVERIES EELATIYE TO HEAT. 



In the whole range of experimental science there is no fact more familiar, 

 or longer known, than the development of Heat by friction. The most ig- 

 norant savage is acquainted with it, it was probably known to the first gen- 

 eration of mankind. Yet, familiar as it is, the science of which it is the germ 

 dates back but a very few years. It was known from the time of Black, 

 that heat disappeared in producing certain changes of state in bodies, and 

 re-appeared when the order of those changes was reversed ; and that the 

 amount of heat, thus converted, had a given relation to the effect produced. 

 In one of these changes namely, evaporation a definite mechanical force 

 is developed, which is again absorbed when the vapor is restored by pressure 

 to the liquid state. It was, therefore, not unnatural to conjecture, that in all 



17 



