Sentence:  Dionysiodorus...sent a letter "ad superos" after he was dead, from the centre of the earth, to signify what distance the same centre was from the superficies of the same, viz. 42,000 <WSD>stadiums</WSD> […]. 
Meaning of the word marked with <WSD> tags:  A Greek measure of length, being the chief one used for itinerary distances, also adopted by the Romans for nautical and astronomical measurements, equal to 600 Greek or 625 Roman feet, or 125 Roman paces, or to 606 feet, 9 inches.