THE YARD, PENDULUM, AND METRE. 435 



from furnishing its quota of observations to the final or 

 -mean conclusion. And the influence of this, it should 

 be observed, is not self-compensating as that of local 

 inequalities of mere density on land would be, but tells 

 all in one direction. For water being, on the average, 

 not more than one-third the weight of an equal bulk of 

 land (such land as the earth's surface consists of) and 

 only TT of the mean density of the globe, the force of 

 gravity at the surface of the sea is less than at the sea- 

 level on land by the attractive force of as much material 

 taken at twice the specific gravity of water, or at ^-ths 

 that of the globe, as would be required to raise the bot- 

 tom to the surface. Supposing then the difficulty of 

 observing the pendulum at sea overcome, and that the 

 whole surface of the globe were dotted over with stations 

 of observation equally distributed over sea and land, 

 from whose intercomparison it were required to derive 

 the mean co-efficient of terrestrial gravitation, or the mean 

 length of the polar pendulum ; it is evident that the sea 

 stations would everywhere conspire to give a less result 

 than the land. According to Dr Young (Phil. Trans., 

 voL cix., page 93) the attraction of an extensive flat 

 mass of any thickness on a point in the middle of its 

 surface is three times that of a sphere of the same mate- 

 rials having that thickness for its diameter. And from 

 this it is very easy to conclude that, supposing the sea to 

 have a mean depth of four miles (which seems not im- 

 probable) the mean defalcation of gravity at its surface, 

 -due to the deficiency of attracting matter, would be three 

 times the attraction of a sphere four riiles in diameter 



