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dangerous american wilderness to settle aboundary dispute between the estates of william penn and lord baltimore and theirrespective colonies of pennsylvania and maryland。 the result was the famous mason anddixon line； which later took on symbolic importance as the dividing line between the slaveand free states。 （although the line was their principal task； they also contributed severalastronomical surveys； including one of the century鈥檚 most accurate measurements of a degree of meridian鈥攁n achievement that brought them far more acclaim in england than the settlingof a boundary dispute between spoiled aristocrats。）back in europe； maskelyne and his counterparts in germany and france were forced to theconclusion that the transit measurements of 1761 were essentially a failure。 one of theproblems； ironically； was that there were too many observations； which when broughttogether often proved contradictory and impossible to resolve。 the successful charting of avenusian transit fell instead to a little…known yorkshire…born sea captain named james cook；who watched the 1769 transit from a sunny hilltop in tahiti； and then went on to chart andclaim australia for the british crown。 upon his return there was now enough information forthe french astronomer joseph lalande to calculate that the mean distance from the earth tothe sun was a little over 150 million kilometers。 （two further transits in the nineteenthcentury allowed astronomers to put the figure at 149。59 million kilometers； where it hasremained ever since。 the precise distance； we now know； is 149。597870691 millionkilometers。） the earth at last had a position in space。
as for mason and dixon； they returned to england as scientific heroes and； for reasonsunknown； dissolved their partnership。 considering the frequency with which they turn up atseminal events in eighteenth…century science； remarkably little is known about either man。 nolikenesses exist and few written references。 of dixon the dictionary of national biographynotes intriguingly that he was 鈥渟aid to have been born in a coal mine；鈥潯ut then leaves it to thereader鈥檚 imagination to supply a plausible explanatory circumstance； and adds that he died atdurham in 1777。 apart from his name and long association with mason； nothing more isknown。
mason is only slightly less shadowy。 we know that in 1772； at maskelyne鈥檚 behest； heaccepted the mission to find a suitable mountain for the gravitational deflectionexperiment； at length reporting back that the mountain they needed was in the central scottishhighlands； just above loch tay； and was called schiehallion。 nothing； however； wouldinduce him to spend a summer surveying it。 he never returned to the field again。 his nextknown movement was in 1786 when； abruptly and mysteriously； he turned up in philadelphiawith his wife and eight children； apparently on the verge of destitution。 he had not been backto america since pleting his survey there eighteen years earlier and had no known reasonfor being there； or any friends or patrons to greet him。 a few weeks later he was dead。
with mason refusing to survey the mountain； the job fell to maskelyne。 so for four monthsin the summer of 1774； maskelyne lived in a tent in a remote scottish glen and spent his daysdirecting a team of surveyors； who took hundreds of measurements from every possibleposition。 to find the mass of the mountain from all these numbers required a great deal oftedious calculating； for which a mathematician named charles hutton was engaged。 thesurveyors had covered a map with scores of figures； each marking an elevation at some pointon or around the mountain。 it was essentially just a confusing mass of numbers； but huttonnoticed that if he used a pencil to connect points of equal height； it all became much moreorderly。 indeed； one could instantly get a sense of the overall shape and slope of the mountain。
he had invented contour lines。
extrapolating from his schiehallion measurements； hutton calculated the mass of the earthat 5；000 million million tons； from which could reasonably be deduced the masses of all theother major bodies in the solar system； including the sun。 so from this one experiment welearned the masses of the earth； the sun； the moon； the other planets and their moons； and gotcontour lines into the bargain鈥攏ot bad for a summer鈥檚 work。
not everyone was satisfied with the results； however。 the shorting of the schiehallionexperiment was that it was not possible to get a truly accurate figure without knowing theactual density of the mountain。 for convenience； hutton had assumed that the mountain hadthe same density as ordinary stone； about 2。5 times that of water； but this was little more thanan educated guess。
one improbable…seeming person who turned his mind to the matter was a country parsonnamed john michell； who resided in the lonely yorkshire village of thornhill。 despite hisremote and paratively humble situation； michell was one of the great scientific thinkers ofthe eighteenth century and much esteemed for it。
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 b