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The second edition of Drawing the Line: How Mason and Dixon Surveyed the Most Famous Border in America updates Edwin Danson's definitive history of the creation of the Mason - Dixon Line to reflect new research and archival documents that have come to light in recent years.

Babcock and Danson discuss a brief history of the Pennsylvania State House Observatory. For the American Philosophical Society's observations in Philadelphia, a timber-framed astronomical observatory was erected in the Pennsylvania State House yard. The observatory was employed again to observe the transit of Mercury in November of the same year. The location of the site for the observatory was not recorded and there is no convincing surviving evidence of its location or of any remains. The Pennsylvania State House, now Independence Hall, was constructed on a 396-foot by 255-foot lot of ground on Chestnut Street between Fifth and Sixth Streets at what was then the edge of the city. Building commenced in 1732 and continued over a period of some 20 years.

At the start of the 18th century, no one knew with any certainty the shape of the earth. Was our planet a perfect sphere or slightly squashed as Newton proposed? Master-surveyor and bestselling author Edwin Danson here presents the stories of the scientists and scholars who cut their way through jungles, crossed the artic tundra, and braved the world's highest mountains to discover the truth about our Earth. A spell-binding scientific adventure story, Weighing the World will intrigue anyone curious about the shape of our planet and how we have come to know it.

Charles Mason made some preliminary longitude calculations based upon the Paris observatory's predicted apparent times for the eclipses of Jupiter's moons. He calculated that John Harlan's backyard was 5 hours 12 minutes and 54 seconds west of the longitude of Paris. He was just eight miles out. Every day, Mason and Jeremiah Dixon meticulously recorded the temperature inside the instrument tent and of the air outside. The vibration of the pendulum, the amount the pendulum swung either side of the vertical, was carefully measured using special scales fitted behind the swinging weights. The difference in time kept by the two clocks, day by day, was determined as usual by equal altitudes observed in winter temperatures that dropped below ten degrees Fahrenheit. Mason and Dixon calculated a preliminary value for a degree of latitude using their measurements along the boundary lines from Middle Point to the Brandywine.

In the comparative shelter of their cold, damp tent, Charles Mason and Jeremiah Dixon computed the logarithms for the spherical trigonometry. They worked out that an angle of 14 minutes and 14 seconds south of their last direction should put them back on course for the next section. This tiny angle was set out on the ground as usual with a long thin triangle and the transit instrument aligned along its great circle side. Equal altitudes were observed with the transit instrument to calibrate their watch in preparation for setting out the next section. The end of the eclipse was observed by Mason through their brass reflecting telescope while Dixon estimated the moment through the quadrant. Before making any precise angular measurement, the instrument had first to be checked for index error by making sure the split view through the sight and the mirrors were in coincidence.

During the recent Pontiac uprising and the on‐going tensions along the frontier, Thomas Cresap's plantation had seen its share of violence, and he knew personally many of the families who had met terrible ends at the hands of the angry Indians. Charles Mason and Jeremiah Dixon were about to lead their team of men into the very forest homes of the native Americans and they needed as much information as possible. Mason would have set up the zenith sector to observe latitude, but either the ground was unsuitable or he was confident from experience that the great circle offset method was safe enough. In modern instruments, the sighting hairs in the eyepiece can be softly illuminated so that they are clearly visible against the black night sky. The wires in Mason and Dixon's instruments were illuminated by someone holding a small spirit lamp or candle near the object lens.

The longitude of Fenwick Island was added to that of the State House observatory in Philadelphia, and David Rittenhouse's home in Norriton, Pennsylvania, derived by the eclipses of Jupiter's moons, also observed at Greenwich. In this work, the first scientific transcontinental geographical connection of its kind, the longitude of British America was solidly tied to that of Greenwich. John Bird's six‐foot instrument, used in North America, had shown remarkable accuracy in the hands of an expert, Mr. Charles Mason. With funds provided by the Royal Society, Nevil Maskelyne had Bird refurbish the zenith sector he had used on Saint Helena and fit it with the new plummet system. The instruments Mason carried north with him included two barometers of Mr Luc's new construction, a theodolite, and, probably, a Hadley quadrant. In his reconnaissance around the foot of the mountain, Mason identified likely locations for the experiment's astronomical observatories.

The astronomer had been appointed by the Board of Longitude to test the reliability and accuracy of John Harrison's chronometer. The testing and astronomical observations had gone on for several weeks until Harrison, and the precious H4 chronometer, departed for England, leaving Nevil Maskelyne behind to complete his own longitude observations. Departing from the Brandywine, Charles Mason headed west towards the town of Lancaster, some thirty‐five miles distant. Mason was referring to the Paxton Boys’ massacre of the Conestoga. Drunk on liquor and inspired by a vague religious fervor, the Paxton Boys’ motive was one of the worst kinds of bigotry, coupled with a loathing for any degree of authority over their wild lives. Tacit support for their bloody crimes came principally from the Presbyterian populations along the Susquehanna, but many across the province were frightened of Indian uprisings and of rumors of Indian atrocities committed in distant, unspecified places.

When Nevil Maskelyne, the seaman's astronomer, succeeded to the esteemed position of Astronomer Royal in 1765, his first priority had been to publish the Nautical Almanac, so essential for mariners. Regardless of his own convictions that the lunar distance method was the most reliable and accessible way for the majority of mariners to find longitude, Maskelyne undertook the evaluation of John Harrison's timepieces methodically, and reasonably impartially. The champions of Harrison's invention were more than sufficient, and vocal enough, to ensure the chronometer would succeed. To aid their navigation of the Mississippi and Missouri rivers, and conduct their mapping exploits, the astronomer Andrew Ellicott instructed Meriwether Lewis in the art of finding latitude by quadrant and longitude by lunar distances using the tables prepared and proved by Charles Mason. Mason and Jeremiah Dixon's contribution to defining the size and shape of Earth was an outstanding achievement.