ON THE MEASUKEMENT OF THE LUNAR DISTURBANCE OF GRAVITY. 119 



3. On Gravitational Observatories. 



In the preceding sections estimates have been made of the amount 

 of distortion -which the upper strata of the earth probably undergo, 

 from the shifting weights corresponding to barometric and tidal oscil- 

 lations. These results appear to me to have an important bearing on the 

 probable 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 computa- 

 tion. 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 satis- 

 factory account could be given. Although I 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 gravitational instrument, capable, 

 let us say, of recording variations of level amounting to 1" or 2". If 

 barometric pi-essure, tidal pressure, and the direct action of the sun and 

 moon, combined together to make apparent slope in one direction, then 

 at an observatory remote from the sea-shore, that slope might perhaps 

 amount to a quarter of a second of arc. Such a disturbance 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 systematic 

 observations, after the manner of the Italians, of the seismic and slower 

 quasi-seismic variations of level. 



I venture to predict that at some future time practical astronomers 

 will no longer be content to eliminate variations of level merely by taking 

 means of results, but will regard corrections derived from a special in- 

 strument as necessary to each astronomical observation. 



