( ( In 1947 ) ( , ( Blackett ( ( ( ( ( ( introduced ( a theory ) ) ( to ( account ( for ( ( the ( Earth 's ) ) ( magnetic field ) ) ) ) ) ) ( as ( ( a function ) ( of ( its rotation ) ) ) ) ) , ) ( with ( ( ( ( the hope ) ( that ( it ( would ( unify ( both ( the ( electromagnetic force ) ) ) ) ) ) ) ) and ) ( ( the force ) ( of gravity ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (CD 1947))) (, ,) (NP (NNP Blackett)) (VP (VBD introduced) (NP (DT a) (NN theory)) (PP (TO to) (NP (NP (NN account)) (PP (IN for) (NP (NP (DT the) (NN Earth) (POS 's)) (JJ magnetic) (NN field))))) (PP (IN as) (NP (NP (DT a) (NN function)) (PP (IN of) (NP (PRP$ its) (NN rotation))))) (, ,) (PP (IN with) (NP (NP (NP (DT the) (NN hope)) (SBAR (IN that) (S (NP (PRP it)) (VP (MD would) (VP (VB unify) (NP (PDT both) (DT the) (JJ electromagnetic) (NN force))))))) (CC and) (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NN gravity))))))) (. .)))
In 1947, Blackett introduced a theory to account for the Earth's magnetic field as a function of its rotation, with the hope that it would unify both the electromagnetic force and the force of gravity.
(ROOT (S (NP (NNP Gravity) (CC and) (NNP magnetism)) (VP (VBP are) (NP (DT both) (JJ common) (NNS forces))) (. .)))
Gravity and magnetism are both common forces.