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among a great deal else； he perceived the wavelike nature of earthquakes； conducted muchoriginal research into magnetism and gravity； and； quite extraordinarily； envisioned thepossibility of black holes two hundred years before anyone else鈥攁 leap of intuitive deductionthat not even newton could make。 when the german…born musician william herscheldecided his real interest in life was astronomy； it was michell to whom he turned forinstruction in making telescopes； a kindness for which planetary science has been in his debtever since。
4but of all that michell acplished； nothing was more ingenious or had greater impactthan a machine he designed and built for measuring the mass of the earth。 unfortunately； hedied before he could conduct the experiments and both the idea and the necessary equipmentwere passed on to a brilliant but magnificently retiring london scientist named henrycavendish。
cavendish is a book in himself。 born into a life of sumptuous privilege鈥攈is grandfatherswere dukes； respectively； of devonshire and kent鈥攈e was the most gifted english scientistof his age； but also the strangest。 he suffered； in the words of one of his few biographers；from shyness to a 鈥渄egree bordering on disease。鈥潯ny human contact was for him a source ofthe deepest disfort。
once he opened his door to find an austrian admirer； freshly arrived from vienna； on thefront step。 excitedly the austrian began to babble out praise。 for a few moments cavendishreceived the pliments as if they were blows from a blunt object and then； unable to takeany more； fled down the path and out the gate； leaving the front door wide open。 it was somehours before he could be coaxed back to the property。 even his housekeeper municatedwith him by letter。
although he did sometimes venture into society鈥攈e was particularly devoted to the weeklyscientific soir茅es of the great naturalist sir joseph banks鈥攊t was always made clear to theother guests that cavendish was on no account to be approached or even looked at。 thosewho sought his views were advised to wander into his vicinity as if by accident and to 鈥渢alk as4in 1781 herschel became the first person in the modern era to discover a planet。 he wanted to call it george；after the british monarch； but was overruled。 instead it became uranus。
it were into vacancy。鈥潯f their remarks were scientifically worthy they might receive amumbled reply； but more often than not they would hear a peeved squeak （his voice appearsto have been high pitched） and turn to find an actual vacancy and the sight of cavendishfleeing for a more peaceful corner。
his wealth and solitary inclinations allowed him to turn his house in clapham into a largelaboratory where he could range undisturbed through every corner of the physical sciences鈥攅lectricity； heat； gravity； gases； anything to do with the position of matter。 the secondhalf of the eighteenth century was a time when people of a scientific bent grew intenselyinterested in the physical properties of fundamental things鈥攇ases and electricity inparticular鈥攁nd began seeing what they could do with them； often with more enthusiasm thansense。 in america； benjamin franklin famously risked his life by flying a kite in an electricalstorm。 in france； a chemist named pilatre de rozier tested the flammability of hydrogen bygulping a mouthful and blowing across an open flame； proving at a stroke that hydrogen isindeed explosively bustible and that eyebrows are not necessarily a permanent feature ofone鈥檚 face。 cavendish； for his part； conducted experiments in which he subjected himself tograduated jolts of electrical current； diligently noting the increasing levels of agony until hecould keep hold of his quill； and sometimes his consciousness； no longer。
in the course of a long life cavendish made a string of signal discoveries鈥攁mong muchelse he was the first person to isolate hydrogen and the first to bine hydrogen and oxygento form water鈥攂ut almost nothing he did was entirely divorced from strangeness。 to thecontinuing exasperation of his fellow scientists； he often alluded in published work to theresults of contingent experiments that he had not told anyone about。 in his secretiveness hedidn鈥檛 merely resemble newton； but actively exceeded him。 his experiments with electricalconductivity were a century ahead of their time； but unfortunately remained undiscovereduntil that century had passed。 indeed the greater part of what he did wasn鈥檛 known until thelate nineteenth century when the cambridge physicist james clerk maxwell took on the taskof editing cavendish鈥檚 papers； by which time credit had nearly always been given to others。
among much else； and without telling anyone； cavendish discovered or anticipated the lawof the conservation of energy； ohm鈥檚 law； dalton鈥檚 law of partial pressures； richter鈥檚 lawof reciprocal proportions； charles鈥檚 law of gases； and the principles of electricalconductivity。 that鈥檚 just some of it。 according to the science historian j。 g。 crowther； he alsoforeshadowed 鈥渢he work of kelvin and g。 h。 darwin on the effect of tidal friction on slowingthe rotation of the earth； and larmor鈥檚 discovery； published in 1915； on the effect of localatmospheric cooling 。 。 。 the work of pickering on freezing mixtures； and some of the work ofrooseboom on heterogeneous equilibria。鈥潯inally； he left clues that led directly to thediscovery of the group of elements known as the noble gases； some of which are so elusivethat the last of them wasn鈥檛 found until 1962。 but our interest here is in cavendish鈥檚 lastknown experiment when in the late summer of 1797； at the age of sixty…seven； he turned hisattention to the crates of equipment that had been left to him鈥攅vidently out of simplescientific respect鈥攂y john michell。
when assembled； michell鈥檚 apparatus looked like nothing so much as an eighteenth…century version of a nautilus weight…training machine。 it incorporated weights；counterweights； pendulums； shafts； and torsion wires。 at the heart of the machine were two350…pound lead balls； which were suspended beside two smaller spheres。 the idea was tomeasure the gravitational deflection of the smaller spheres by the larger ones； which would allow the first measurement of the elusive force known as the gravitational constant； and fromwhich the weight （strictly speaking； the mass）5of the earth could be deduced。
because gravity holds planets in orbit and makes falling objects land with a bang； we tendto think of it as a powerful force； but it is not really。 it is only powerful in a kind of collectivesense； when one massive object； like the sun； holds on to another massive object； like theearth。 at an elemental level gravity is extraordinarily unrobust。 each time you pick up a bookfrom a table or a dime from the floor you effortlessly overe the bined gravitationalexertion of an entire planet。 what cavendish was trying to do was measure gravity at thisextremely featherweight level。
delicacy was the key word。 not a whisper of disturbance could be allowed into the roomcontaining the apparatus； so cavendish took up a position in an adjoining room and made hisobservations with a telescope aimed through a peephole。 the work was incredibly exactingand involved seventeen delicate； interconnected measurements； which together took nearly ayear to plete。 when at last he had finished his calculations； cavendish announced that theearth weighed a little over 13；000；000；000；000；000；000；000 pounds； or six billion trillionmetric tons； to use the modern measure。 （a metric ton is 1；000 kilograms or 2；205 pounds。）today； scientists have at their disposal machines so precise they can detect the weight of asingle bacterium and so sensitive that readings can be disturbed by someone yawning seventy…five feet away； but they have not significantly improved on cavendish鈥檚 measurements of1797。 the current best estimate for earth鈥檚 weight is 5。9725 billion trillion metric tons； adifference of only about 1 percent from cavendish鈥檚 finding。 interestingly； all of this merelyconfirmed estimates made by newton 110 years before cavendish without any experimentalevidence at all。
so； by the late eighteenth century scientists knew very precisely the shape and dimensionsof the earth and its distance from the sun and planets； and now cavendish； without evenleaving home； had given them its weight。 so you might think that determining the age of theearth would be relatively straightforward。 after all； the necessary materials were literally attheir feet。 but no。 human beings would split the atom and invent television； nylon； and instantcoffee before they could figure out the age of their own planet。
to understand why； we must travel north to scotland and begin with a brilliant and genialman； of whom few have ever heard； who had just invented a new science called geology。
5to a physicist； mass and weight are two quite different things。 your mass stays the same wherever you go； butyour weight varies depending on how far you are from the center of some other massive object like a planet。
travel to the moon and you will be much lighter but no less massive。 on earth； for all practical purposes； massand weight are the same and so the terms can be treated as synon