72 Geological Society of Dublin. 



lectures last summer, if not at an earlier date. The conversation in 

 which I was engaged with you on this and similar subjects, at the 

 last anniversary dinner of the Geological Society, interested and 

 excited me at the time very much indeed ; and if you think the fore- 

 going memorandum, which I have drawn up at your desire, worthy 

 of being incorporated in any communication of your own to the 

 Society, it is perfectly at your service for that purpose. 



" I remain, my dear Sir, very truly yours, 



" William R. Hamilton." 



Mr. Mallet would remind the Society, that the earth's surface was 

 the medium through which two great waves of heat were continually 

 transmitted ; one proceeding from the sun, by part of which the at- 

 mosphere was heated, and the other coming from the interior of the 

 earth towards the surface. In every climate there was a plane lying 

 between the influences of these two waves, which never changed its 

 temperature, and which varied in its depth below the earth's sur- 

 face. As there was thus a transmission of heat from the interior of 

 the earth towards its surface, and also from the surface towards the 

 centre, the plane of constant temperature, or isogeothermal plane as 

 it was called, would be found in any given locality at a determinate 

 depth ; and if the supplies of external and internal heat were con- 

 stant, it would always be found at the same depth in the same place. 

 But inasmuch as the earth's surface was exposed to temperatures, 

 varying with the winter and summer seasons, the level of that plane 

 must rise and fall in proportion to the force of the variable wave. 

 The average depth of this plane in their latitudes was about sixty 

 feet, but was far greater in the tropical climates, in some of which 

 the heating power of the sun ranged to a depth of nearly 500 feet 

 below the earth's surface. 



The result of the varying intensity of the external wave of heat 

 must be the contraction and expansion of the earth's crust due to 

 the difference between the temperatures of summer and winter, and 

 hence a certain annual motion in the earth's crust, besides which, 

 from similar causes, acting in shorter periods, and in less degree, 

 there must be a diurnal motion. To measure this systematically, 

 would be, it appeared to him, highly important, not perhaps directly 

 to those more obvious parts of geology which treat of the elevation 

 of mountain ranges and the depression of valleys, and so forth ; but 

 inasmuch as it would lead to a region of investigation which was at 

 present absolutely unknown ; and by penetrating which it would 

 probably be found in this, as in so many other parts of terrestrial 

 physics, that forces and motions the most minute, and scarcely to be 

 detected, when taken in aggregate were essential and potent parts 

 of the universal machine. There had yet been no investigation of 

 the rate of expansion of any rock, the tables of the rate of expan- 

 sion of the very few solids hitherto published having reference <^dy 

 to substances which were of value to the astronomer or the engineer. 

 Therefore with respect to the measurement of these motions, the fa- 

 cility with which the observations necessary for that end could be 



