▁the ▁Earth ▁itself ▁is ▁magnetic . ▁In ▁ 1 6 8 7 ▁Isaac ▁Newton ▁published ▁his ▁Princi p ia , ▁which ▁not ▁only ▁laid ▁the ▁found ations ▁for ▁classical ▁mechan ics ▁and ▁grav itation ▁but ▁also ▁explained ▁a ▁variety ▁of ▁ge oph ys ical ▁phen omena ▁such ▁as ▁t ides ▁and ▁the ▁pre cess ion ▁of ▁the ▁equ ino x . ▁ ▁These ▁experimental ▁and ▁mathematical ▁anal ys es ▁were ▁applied ▁to ▁several ▁areas ▁of ▁ge oph ys ics : ▁Earth ' s ▁shape , ▁density , ▁and ▁gravity ▁field ▁( Pierre ▁B oug uer , ▁Alex is ▁Cla ira ut ▁and ▁Henry ▁Cav end ish ), ▁Earth ' s ▁magnetic ▁field ▁( Alex ander ▁von ▁H umb old t , ▁Edmund ▁Hal ley ▁and ▁Carl ▁Friedrich ▁G auss ), ▁se ism ology ▁( John ▁Mil ne ▁and ▁Robert ▁Mal let ), ▁and ▁the ▁Earth ' s ▁age , ▁heat ▁and ▁radio activity ▁( Ar thur ▁Hol mes ▁and ▁William ▁Thom son , ▁ 1 st ▁Baron ▁Kel vin ). ▁ ▁There ▁are ▁several ▁descri ptions ▁and ▁discuss ions ▁about ▁a ▁philosoph ical ▁theory ▁of ▁the ▁water ▁cycle ▁by ▁Marcus ▁Vit ru vi us , ▁Leon ardo ▁da ▁V inci ▁and ▁Bernard ▁Pal iss y . ▁P ione ers ▁in ▁hydro log y ▁include ▁Pierre ▁Per ra ult , ▁Ed me ▁Mari otte ▁and ▁Edmund ▁Hal ley ▁in ▁studies ▁of ▁such ▁things ▁as ▁ra inf all , ▁run off , ▁d rain age ▁area , ▁velocity , ▁river ▁cross - section ▁measurements ▁and