Comparison of the acceleration due to gravity at the National Physical Laboratory, Teddington, the Bureau International des Poids et Mesures, Sèvres, the Physikalisch-Technische Bundesanstalt, Brunswick, and the Geodetic Institute, Potsdam

The values of gravity at these stations have been compared by means of pendulum observations with Invar invariable pendulums. The observed differences of gravity from the National Physical Laboratory are: $ \matrix\format\l\kern.8em&\l \\ \text{B.I.P.M.} & -256\cdot 73\pm 0\cdot 49\,\text{mgal} \\ \text{P.T.B.} & +\ \,68\cdot 68\pm 0\cdot 49\,\text{mgal} \\ \text{Bad Harzburg} & -\ \,15\cdot 68\pm 0\cdot 49\,\text{mgal} \endmatrix $ The accuracy of the measurements is not so great as has been achieved once or twice previously with the same apparatus, mainly because the changes in the lengths of the pendulums were greater than usual. These differences have been combined with German pendulum observations and with gravimeter comparisons with the following results: Value of gravity at N.P.L. on the Potsdam system: 981196$\cdot $29 $\pm $ 0$\cdot $3 mgal. Differences between sites of absolute determinations of gravity: $\aligned \text{N.P.L.}-\text{B.I.P.M.}\quad +256\cdot 45\pm 0\cdot 3\,\text{mgal}\\ \text{N.P.L.}-\text{P.T.B.}\quad \quad -\ \,68\cdot 98\pm 0\cdot 3\,\text{mgal}\\ \text{P.T.B.}-\text{Potsdam}\quad -\ \ 8\cdot 95\pm \, 0\cdot 4\,\text{mgal}\\ (g\ \text{at Potsdam}=981274\,\text{mgal}.)\quad \quad \endaligned $ (g at Potsdam = 981274 mgal.) The effects of these results on gravity surveys based on Cambridge and on the absolute value of gravity are indicated.