82 Dr. Robinson on the Annual Motions of the Earth's Crust. 



season of greatest cold. At first it was derived from direct 

 and reflected observations of stars, latterly from those of the 

 lines of the telescope ; and each of these columns is based on 

 more than 2000 observations. 



The fourth gives the mean temperature of each month as 

 shown by a standard thermometer in a northern aspect, noted 

 at 10 o'clock in the morning and evening. 



Besides these the transit instrument is disturbed in azimuth 

 when any sudden change of temperature occurs, and probably 

 also with an annual period; of this however I have seldom 

 obtained measures, as whenever observed it is corrected by 

 the meridian mark. Cold makes the western pivot move to 

 the south. 



On inspecting the table, it is evident that both these move- 

 ments depend on the temperature, having their maxima and 

 minima at the times of greatest heat and cold; the second 

 exactly, the first less precisely, but still with a decided ap- 

 proximation. Nor do they vary according to the same law, 

 which may probably arise from some secondary force acting 

 more powerfully in one direction than the other. Can it be 

 the infiltration of water into the subjacent strata? When 

 sinking a well many years since, no water was found till the 

 limestone (which under the observatory is covered by about 

 100 feet of clay) was bored through, when it rose with great 

 violence about 40 feet. The hydrostatic pressure, varying 

 with the supply at different times, seems capable of such an 

 action. The motion of the western point is such as might be 

 explained in kind, though not perhaps in magnitude, by Mr. 

 Mallet's hypothesis of a general expansion, and consequently 

 increased curvature of the surface of Ireland; but that of the 

 northern seems to me exactly the reverse, unless we assume 

 that the expansion of the trap rocks which predominate in 



