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 synonymous。 at least outside the classroom。
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5 THE STONE…BREAKERS
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at just the time that henry cavendish was pleting his experiments in london； fourhundred miles away in edinburgh another kind of concluding moment was about to take placewith the death of james hutton。 this was bad news for hutton； of course； but good news forscience as it cleared the way for a man named john playfair to rewrite hutton鈥檚 work withoutfear of embarrassment。
hutton was by all accounts a man of the keenest insights and liveliest conversation； a delightin pany； and without rival when it came to understanding the mysterious slow processesthat shaped the earth。 unfortunately； it was beyond him to set down his notions in a form thatanyone could begin to understand。 he was； as one biographer observed with an all but audiblesigh； 鈥渁lmost entirely innocent of rhetorical acplishments。鈥潯early every line he pennedwas an invitation to slumber。 here he is in his 1795 masterwork； a theory of the earth withproofs and illustrations ； discussing 。 。 。 something：
the world which we inhabit is posed of the materials； not of the earth whichwas the immediate predecessor of the present； but of the earth which； in ascendingfrom the present； we consider as the third； and which had preceded the land thatwas above the surface of the sea； while our present land was yet beneath the waterof the ocean。
yet almost singlehandedly； and quite brilliantly； he created the science of geology andtransformed our understanding of the earth。 hutton was born in 1726 into a prosperousscottish family； and enjoyed the sort of material fort that allowed him to pass much of hislife in a genially expansive round of light work and intellectual betterment。 he studiedmedicine； but found it not to his liking and turned instead to farming； which he followed in arelaxed and scientific way on the family estate in berwickshire。 tiring of field and flock； in1768 he moved to edinburgh； where he founded a successful business producing salammoniac from coal soot； and busied