▁tried ▁to ▁measure ▁the ▁drag ging ▁coefficient ▁within ▁glass . ▁Con tr ary ▁to ▁F ize au ▁he ▁used ▁a ▁rot ating ▁device ▁so ▁he ▁found ▁the ▁same ▁effect ▁as ▁S agn ac . ▁While ▁Har ress ▁himself ▁misunder stood ▁the ▁meaning ▁of ▁the ▁result , ▁it ▁was ▁shown ▁by ▁La ue ▁that ▁the ▁theoretical ▁explanation ▁of ▁Har ress ' ▁experiment ▁is ▁in ▁accord ance ▁with ▁the ▁S agn ac ▁effect . ▁Event ually , ▁the ▁Michel son – G ale – P ear son ▁experiment ▁( 1 9 2 5 , ▁a ▁variation ▁of ▁the ▁S agn ac ▁experiment ) ▁indicated ▁the ▁angular ▁velocity ▁of ▁the ▁Earth ▁itself ▁in ▁accord ance ▁with ▁special ▁relativ ity ▁and ▁a ▁rest ing ▁a ether . ▁ ▁Rel ativity ▁of ▁simultane ity ▁The ▁first ▁deriv ations ▁of ▁relativ ity ▁of ▁simultane ity ▁by ▁synchron ization ▁with ▁light ▁signals ▁were ▁also ▁simplified . ▁Daniel ▁F rost ▁Com stock ▁( 1 9 1 0 ) ▁placed ▁an ▁observer ▁in ▁the ▁middle ▁between ▁two ▁clock s ▁A ▁and ▁B . ▁From ▁this ▁observer ▁a ▁signal ▁is ▁sent ▁to ▁both ▁clock s , ▁and ▁in ▁the ▁frame ▁in ▁which ▁A ▁and ▁B ▁are ▁at ▁rest , ▁they ▁synchron ously ▁start ▁to ▁run . ▁But ▁from ▁the ▁perspective ▁of ▁a ▁system ▁in ▁which ▁A ▁and ▁B ▁are ▁moving , ▁clock ▁B ▁is ▁first ▁set ▁in ▁motion , ▁and ▁then ▁comes ▁clock ▁A   – ▁so ▁the ▁clock s ▁are ▁not ▁synchron ized . ▁Also ▁Ein stein ▁( 1 9 1 7