304 Royal Society :•— 



Dr. Tyndall's statement, that " the hazy surfaces produced by the com- 

 pression of the mass were observed to be in a state of intense commo- 

 tion, which followed closely upon the edge of the surface as it advanced 

 through the solid. It is finally shown that these surfaces are due to 

 tlie liquefaction of the ice in planes perpendicular to the pressure." 



There can be no doubt but that the "oscillations" in the melting- 

 point of ice, and the distinction between strong and weak pieces in 

 this respect, described by Dr. Tyndall in the second section of his 

 paper, are consequences of the varying pressures which different por- 

 tions of a mass of ice must experience when portions within it 

 become liquefied. 



The elevation of the melting temperature which my brother's 

 theory shows must be produced by diminishing the pressure of ice 

 below" the atmospheric pressure, and to which I alluded as a subject 

 for experimental illustration, in the article describing my experi- 

 mental demonstration of the lowering effect of pressure (Proceedings, 

 E,oy. Soc. Edinb. Feb. 1850), demonstrates that a vesicle of water 

 cannot form in the interior of a solid of ice excej)! at a temperature 

 higher than 0° Cent. This is a conclusion which Dr. Tyndall ex- 

 presses as a result of mechanical considerations : thus, " Regarding 

 heat as a mode of motion," "liberty of liquidity is attamed by the 

 molecules at the surface of a mass of ice before the molecules at the 

 centre of the mass can attain this liberty." 



The physical theory shows that a removal of the atmospheric 

 pressure would raise the melting-point of ice by j-foths of a degree 

 Centigrade. Hence it is certain that the iuteiior of a solid of ice, 

 heated by the condensation of solar rays by a lens, will rise to at 

 least that* excess of temperature above the superficial parts. It appears 

 verv nearly certain that cohesion will prevent the evolution of a 

 bubble of vapour of water in a vesicle of water forming by this process 

 in the interior of a mass of ice, until a high " negative pressure" has 

 been reached, that is to say, until cohesion has been called largely 

 into operation, especially if the water and ice contain little or no air 

 by absorption (just as water freed from air may be raised consider- 

 ably above its boiling-point under any non- evanescent hydrostatic 

 pressure). Hence it appears nearly certain that the interior of a 

 block of ice originally clear, and made to possess vesicles of water by 

 the concentration of radiant heat, as in the beautiful experiments 

 described by Dr. Tyndall in the commencement of his paper, will 

 rise very considerably in temperature, while the vesicles enlarge under 

 the continued influence of the heat received by radiation through the 

 cooler enveloping ice and through the fluid medium (air and a watery 

 film, or water) touching it all round, which is necessarily at 0° Cent, 

 where it touches the solid. 



I find I have not time to execute my intention of sending you to- 

 day a physical explanation of the blue veins of glaciers which occurred 

 to me last May, but I hope to be able to send it in a short time. 



Jan. 21, 1858. William Thomson. 



" On the Practical Use of the Aneroid Barometer as an Orometer." 



By Capt.W.S.Moorsom, Member of the Institution of Civil Engineers. 



A Government Commission to Ceylon in the begimiing of 1857, 



