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around the globe鈥攖o siberia； china； south africa； indonesia； and thewoods of wisconsin； among many others。 france dispatched thirty…two observers； britaineighteen more； and still others set out from sweden； russia； italy； germany； ireland； andelsewhere。
it was history鈥檚 first cooperative international scientific venture； and almost everywhere itran into problems。 many observers were waylaid by war； sickness； or shipwreck。 others madetheir destinations but opened their crates to find equipment broken or warped by tropical heat。
once again the french seemed fated to provide the most memorably unlucky participants。
jean chappe spent months traveling to siberia by coach； boat； and sleigh； nursing his delicateinstruments over every perilous bump； only to find the last vital stretch blocked by swollen3the next transit will be on june 8； 2004； with a second in 2012。 there were none in the twentieth century。
rivers； the result of unusually heavy spring rains； which the locals were swift to blame on himafter they saw him pointing strange instruments at the sky。 chappe managed to escape withhis life； but with no useful measurements。
unluckier still was guillaume le gentil； whose experiences are wonderfully summarizedby timothy ferris in ing of age in the milky way 。 le gentil set off from france a yearahead of time to observe the transit from india； but various setbacks left him still at sea on theday of the transit鈥攋ust about the worst place to be since steady measurements wereimpossible on a pitching ship。
undaunted； le gentil continued on to india to await the next transit in 1769。 with eightyears to prepare； he erected a first…rate viewing station； tested and retested his instruments；and had everything in a state of perfect readiness。 on the morning of the second transit； june4； 1769； he awoke to a fine day； but； just as venus began its pass； a cloud slid in front of thesun and remained there for almost exactly the duration of the transit： three hours； fourteenminutes； and seven seconds。
stoically； le gentil packed up his instruments and set off for the nearest port； but en routehe contracted dysentery and was laid up for nearly a year。 still weakened； he finally made itonto a ship。 it was nearly wrecked in a hurricane off the african coast。 when at last hereached home； eleven and a half years after setting off； and having achieved nothing； hediscovered that his relatives had had him declared dead in his absence and hadenthusiastically plundered his estate。
in parison； the disappointments experienced by britain鈥檚 eighteen scattered observerswere mild。 mason found himself paired with a young surveyor named jeremiah dixon andapparently they got along well； for they formed a lasting partnership。 their instructions wereto travel to sumatra and chart the transit there； but after just one night at sea their ship wasattacked by a french frigate。 （although scientists were in an internationally cooperativemood； nations weren鈥檛。） mason and dixon sent a note to the royal society observing that itseemed awfully dangerous on the high seas and wondering if perhaps the whole thingoughtn鈥檛 to be called off。 in reply they received a swift and chilly rebuke； noting that they hadalready been paid； that the nation and scientific munity were counting on them； and thattheir failure to proceed would result in the irretrievable loss of their reputations。 chastened；they sailed on； but en route word reached them that sumatra had fallen to the french and sothey observed the transit inconclusively from the cape of good hope。 on the way home theystopped on the lonely atlantic outcrop of st。 helena； where they met maskelyne； whoseobservations had been thwarted by cloud cover。 mason and maskelyne formed a solidfriendship and spent several happy； and possibly even mildly useful； weeks charting tidalflows。
soon afterward； maskelyne returned to england where he became astronomer royal； andmason and dixon鈥攏ow evidently more seasoned鈥攕et off for four long and often perilousyears surveying their way through 244 miles of 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 extr