nce in sphericity there； and because they reasoned that mountainswould give them good sightlines。 in fact； the mountains of peru were so constantly lost incloud that the team often had to wait weeks for an hour鈥檚 clear surveying。 on top of that； theyhad selected one of the most nearly impossible terrains on earth。 peruvians refer to theirlandscape as muy accidentado 鈥斺�渕uch accidented鈥濃�攁nd this it most certainly is。 thefrench had not only to scale some of the world鈥檚 most challenging mountains鈥攎ountainsthat defeated even their mules鈥攂ut to reach the mountains they had to ford wild rivers； hacktheir way through jungles； and cross miles of high； stony desert； nearly all of it uncharted andfar from any source of supplies。 but bouguer and la condamine were nothing if nottenacious； and they stuck to the task for nine and a half long； grim； sun…blistered years。
shortly before concluding the project； they received word that a second french team； takingmeasurements in northern scandinavia （and facing notable disforts of their own； fromsquelching bogs to dangerous ice floes）； had found that a degree was in fact longer near thepoles； as newton had promised。 the earth was forty…three kilometers stouter when measuredequatorially than when measured from top to bottom around the poles。
bouguer and la condamine thus had spent nearly a decade working toward a result theydidn鈥檛 wish to find only to learn now that they weren鈥檛 even the first to find it。 listlessly； they pleted their survey； which confirmed that the first french team was correct。 then； still notspeaking； they returned to the coast and took separate ships home。
something else conjectured by newton in the principia was that a plumb bob hung near amountain would incline very slightly toward the mountain； affected by the mountain鈥檚gravitational mass as well as by the earth鈥檚。 this was more than a curious fact。 if youmeasured the deflection accurately and worked out the mass of the mountain； you couldcalculate the universal gravitational constant鈥攖hat is； the basic value of gravity； known asg鈥攁nd along with it the mass of the earth。
bouguer and la condamine had tried this on peru鈥檚 mount chimborazo； but had beendefeated by both the technical difficulties and their own squabbling； and so the notion laydormant for another thirty years until resurrected in england by nevil maskelyne； theastronomer royal。 in dava sobel鈥檚 popular book longitude； maskelyne is presented as a ninnyand villain for failing to appreciate the brilliance of the clockmaker john harrison； and thismay be so； but we are indebted to him in other ways not mentioned in her book； not least forhis successful scheme to weigh the earth。 maskelyne realized that the nub of the problem laywith finding a mountain of sufficiently regular shape to judge its mass。
at his urging； the royal society agreed to engage a reliable figure to tour the british islesto see if such a mountain could be found。 maskelyne knew just such a person鈥攖heastronomer and surveyor charles mason。 maskelyne and mason had bee friends elevenyears earlier while engaged in a project to measure an astronomical event of great importance：
the passage of the planet venus across the face of the sun。 the tireless edmond halley hadsuggested years before that if you measured one of these passages from selected points on theearth； you could use the principles of triangulation to work out the distance to the sun； andfrom that calibrate the distances to all the other bodies in the solar system。
unfortunately； transits of venus； as they are known； are an irregular occurrence。 theye in pairs eight years apart； but then are absent for a century or more； and there were nonein halley鈥檚 lifetime。
3but the idea simmered and when the next transit came due in 1761；nearly two decades after halley鈥檚 death； the scientific world was ready鈥攊ndeed； more readythan it had been for an astronomical event before。
with the instinct for ordeal that characterized the age； scientists set off for more than ahundred locations 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 lo