( ( Historically mass ) ( ( was ( ( defined ( by ( use ( of ( balance scales ) ) ) ) ) ( to ( ( compare ( ( the object ) ( ( whose mass ) ( is ( to ( be ( ( ( measured ( with ( a ( standard mass ) ) ) ) , ) perhaps ) ) ) ) ) ) ) ( by ( use ( of ( an ( accurate ( laboratory balance ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Historically) (NN mass)) (VP (VBD was) (VP (VBN defined) (PP (IN by) (NP (NP (NN use)) (PP (IN of) (NP (NN balance) (NNS scales))))) (S (VP (TO to) (VP (VB compare) (NP (NP (DT the) (NN object)) (SBAR (WHNP (WP$ whose) (NNS mass)) (S (VP (VBZ is) (S (VP (TO to) (VP (VB be) (VP (VBN measured) (PP (IN with) (NP (DT a) (JJ standard) (NN mass))) (, ,) (ADVP (RB perhaps)))))))))) (PP (IN by) (NP (NP (NN use)) (PP (IN of) (NP (DT an) (ADJP (JJ accurate)) (NN laboratory) (NN balance)))))))))) (. .)))
Historically mass was defined by use of balance scales to compare  the object whose mass is to be measured with a standard mass, perhaps by use of an accurate  laboratory balance.
(ROOT (S (NP (DT A) (NN student)) (VP (MD should) (VP (VB use) (NP (DT a) (NN balance) (S (VP (TO to) (VP (VB compare) (NP (NP (DT the) (NNS masses)) (PP (IN of) (NP (CD two) (JJ small) (NNS rocks)))))))))) (. .)))