426 On Variations in the Vertical. 



3. On Gravitational Observatories. 



In the preceding sections estimates have been made of the 

 amount of distortion which the upper strata of the earth pro- 

 bably undergo from the shifting weights corresponding to 

 barometric and tidal oscillations. These results appear to me 

 to have an important bearing on the utility of gravitational 

 observatories. 



It is not probable, at least for many years to come, that the 

 state of tidal and barometric pressure, for a radius of 500 miles 

 round any spot on the earth's surface, will be known with 

 sufficient accuracy to make even a rough approximation to the 

 slope of the surface a possibility. And were these data known, 

 the heterogeneity of geological strata would form a serious 

 obstacle to the possibility of carrying out such a computation. 

 It would do little in relieving us from these difficulties to place 

 the observatory at the bottom of a mine. 



Accordingly the prospect of determining experimentally 

 the lunar disturbance of gravity appears exceedingly remote ; 

 and I am compelled reluctantly to conclude that continuous 

 observations with gravitational instruments of very great 

 delicacy are not likely to lead to results of any great interest. 

 It appears likely that such an instrument, even in the most 

 favourable site, would record incessant variations of which no 

 satisfactory account could be given. Although 1 do not 

 regard it as probable that such a delicate instrument should be 

 adopted for regular continuous observations, yet, by choosing 

 a site where the flexure of the earth's surface is likely to be 

 great, it is conceivable that a rough estimate might be made 

 of the average modulus of elasticity of the upper strata of the 

 earth for one or two hundred miles from the surface. 



These conclusions, which I express with much diffidence, 

 are by no means adverse to the utility of a coarser gravita- 

 tional instrument, capable, let us say, of recording variations 

 of level amounting to 1" or 2". If barometric pressure, tidal 

 pressure, and the direct action of the sun and moon combine 

 together to make apparent slope in one direction, then, at an 

 observatory remote from the sea-shore, that slope might per- 

 haps amount to a quarter of a second of arc. Such a disturb- 

 ance of level would not be important compared with the 

 minimum deviations which could be recorded by the supposed 

 instrument. 



It would then be of much value to obtain continuous syste- 

 matic observations, after the manner of the Italians, of the 

 seismic and slower quasi-seismic variations of level. 



