1 1 4 Chemical and Physical Notes 



force of gravity in the locality, and the occurrence of rocks of 

 relatively low density may cause an analogous deficiency of 

 local gravitational attraction. 



Looking to the great variety of mineral substances which 

 we meet with in the surface crust of the earth, and to the 

 differences of their densities, it is not surprising that recent 

 observations with the pendulum show that local peculiarities 

 of the force of gravity are the rule and not the exception. 

 The differences in neighbouring localities are not usually 

 great, but the pendulum is an instrument of almost infinite 

 delicacy, so that with patience the gravitational map of 

 a district can be correctly constructed, no matter how slight 

 the differences may be. 



It will be evident from the examination of Table XVIII, 

 that by the combined use of the barometer and the hypso- 

 meter we have in our hands the means of determining local 

 deviations of the force of gravity from its normal value. 



Rule for detecting the deviation of the local force of gravity 

 from, the normal by simultaneous observations of the standard 

 height of the barometer and of tJie boiling-point of water. 

 Determine the height of the barometer at o C. Apply 

 the gravitation correction for distance of the locality from 

 the centre of the earth, so as to reduce it to its equivalent 

 height when the mercury is exposed to the attraction of the 

 standard force of gravity, as at the sea-level in latitude 45. 

 Determine the boiling-point of water at the same time and 

 place. Refer to the tables connecting this boiling-point of 

 water with standard barometric pressure, and from them take 

 out the boiling-point corresponding to the observed standard 

 barometric pressure. If the tabular and the observed boiling- 

 points are identical, then the force of gravity at the place 

 is normal. If the tabular boiling-point is higher than the 

 observed boiling-point, then the local force of gravity is less 

 than the normal ; if it is lower than the observed boiling- 

 point, then the local force of gravity is greater than the 

 normal. In arriving at these conclusions, we postulate the 

 complete exactness of the tables giving the relation between 

 barometric pressure and the temperature of saturated steam, 



