How Do You Find Level When Gravity Itself Is Crooked?
Trade Archive
10,770 views
On the night of June 6th, 1992, Copa Airlines Flight 201 leveled off at 25,000 feet on its way from Panama City to Cali. A pinched wire inside the captain's vertical gyro sent intermittent false roll information to his artificial horizon. A transfer switch had both pilots' displays fed from the same faulty gyro, and with no natural horizon in the dark, the 737 rolled past vertical and went down in the Darién jungle. All 47 people on board died. The instrument that failed was doing the oldest job in building: telling someone where level is.
This documentary follows that job from ancient Egypt to the big physics machines of today. It starts with a 12th Dynasty plumb line level and Flinders Petrie's 1883 survey of the Great Pyramid pavement. The Roman architect Vitruvius describes the chorobates, admits that wind can swing its plumb lines, tells builders to pour water into a groove on top instead, and answers an objection from Archimedes that water follows the curve of the Earth.
In 1661, Melchisédech Thévenot sealed a bubble in a slightly curved tube of alcohol and wrote to Huygens and Viviani about it. The video explains why the curve of that glass decides a level's sensitivity, how vials are bent or ground today, and how the bubble moved from telescopes to surveying instruments. It covers the wye and dumpy levels, Gurley of Troy, New York, and the first automatic level. Then it follows the level into the trades: Stanley Rule & Level and its "Patent King," Justus Traut, the Stratton brothers of Greenfield, who made rifles for Springfield Armory before they made levels, Leonard Davis's cast iron inclinometers, and Stabila in Germany.
In the machine shop, South Bend's 1934 manual told machinists that no lathe can do accurate work unless it is level, and that a carpenter's level with a bent tube vial wasn't good enough. The video looks at the Starrett 98 and the 199 master precision level, including why Starrett machines the base of the 98 slightly concave. It covers Quinn Dunki's point that leveling a lathe is really about removing twist, and Wayne Moore's warning that a precision level can pick up the floor, the carriage, and someone walking past the machine. Electronic levels and lasers carry it to 0.2 of an arcsecond.
Every one of those instruments trusts the direction of gravity, and gravity isn't straight. Bouguer measured a mountain pulling a plumb line on Chimborazo in 1738. Maskelyne measured 11.6 seconds of deflection on Schiehallion in 1774, and Charles Hutton turned it into a density for the Earth. In India, Everest and Pratt found the Himalayas pulling less than they should. That led to the geoid, the lumpy surface a spirit level actually follows, and the GRACE satellites that mapped it from orbit.
Height depends on where you measure it from. A bridge between Laufenburg in Switzerland and Laufenburg in Germany came out 54 centimeters off because the two countries measured from different seas and a correction went the wrong way. In New Orleans, the Army Corps of Engineers found outfall canal protection built 1 to 2 feet lower than intended because of a height reference mix-up. The official US height datum is off by about half a meter, and its replacement has been delayed again and again. The video ends at LIGO, whose 4 kilometer arms had to be built straight rather than level and CERN's sloping collider tunnel, and back at Copa 201.
Subscribe to Trade Archive for more stories about the tools, machines and standards behind the trades: @tradearchive
Sources
Althouse, W., L. Hand, L. Jones, A. Lazzarini, and R. Weiss. "Precision Alignment of the LIGO 4 km Arms Using the Dual-Frequency Differential Global Positioning System." LIGO-P000006, 2001.
Dirección de Aeronáutica Civil, Panamá. Final Report, Copa Airlines Flight 201, Boeing 737-204, HP-1205CMP. 1992.
Moore, Wayne R. Foundations of Mechanical Accuracy. Bridgeport, CT: Moore Special Tool Co., 1970.
National Geodetic Survey. "New Datums." National Oceanic and Atmospheric Administration.
Petrie, W. M. Flinders. The Pyramids and Temples of Gizeh. London: Field and Tuer, 1883.
Smallwood, R. M. "Schiehallion, Maskelyne and the density of the Earth." History of Geo- and Space Sciences 9 (2018).
South Bend Lathe Works. How to Run a Lathe. South Bend, IN, 1934.
L.S. Starrett Company. "FAQ: 98 Series Levels."
US Army Corps of Engineers. Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System, Volume II: Geodetic Vertical and Water Level Datums. Interagency Performance Evaluation Task Force, 2006–2009.
Curry, Robert S., Jr. Erecting Means for Gyro Vertical Instruments. US Patent 2,408,411, filed August 16, 1944, and issued October 1, 1946.
Kanton Aargau. "Brücke Laufenburg: Korrektur nötig." Press release, December 24, 2003.
4 Comments
How Do You Find Level When Gravity Itself Is Crooked?
Trade Archive
10,770 views
On the night of June 6th, 1992, Copa Airlines Flight 201 leveled off at 25,000 feet on its way from Panama City to Cali. A pinched wire inside the captain's vertical gyro sent intermittent false roll information to his artificial horizon. A transfer switch had both pilots' displays fed from the same faulty gyro, and with no natural horizon in the dark, the 737 rolled past vertical and went down in the Darién jungle. All 47 people on board died. The instrument that failed was doing the oldest job in building: telling someone where level is.
This documentary follows that job from ancient Egypt to the big physics machines of today. It starts with a 12th Dynasty plumb line level and Flinders Petrie's 1883 survey of the Great Pyramid pavement. The Roman architect Vitruvius describes the chorobates, admits that wind can swing its plumb lines, tells builders to pour water into a groove on top instead, and answers an objection from Archimedes that water follows the curve of the Earth.
In 1661, Melchisédech Thévenot sealed a bubble in a slightly curved tube of alcohol and wrote to Huygens and Viviani about it. The video explains why the curve of that glass decides a level's sensitivity, how vials are bent or ground today, and how the bubble moved from telescopes to surveying instruments. It covers the wye and dumpy levels, Gurley of Troy, New York, and the first automatic level. Then it follows the level into the trades: Stanley Rule & Level and its "Patent King," Justus Traut, the Stratton brothers of Greenfield, who made rifles for Springfield Armory before they made levels, Leonard Davis's cast iron inclinometers, and Stabila in Germany.
In the machine shop, South Bend's 1934 manual told machinists that no lathe can do accurate work unless it is level, and that a carpenter's level with a bent tube vial wasn't good enough. The video looks at the Starrett 98 and the 199 master precision level, including why Starrett machines the base of the 98 slightly concave. It covers Quinn Dunki's point that leveling a lathe is really about removing twist, and Wayne Moore's warning that a precision level can pick up the floor, the carriage, and someone walking past the machine. Electronic levels and lasers carry it to 0.2 of an arcsecond.
Every one of those instruments trusts the direction of gravity, and gravity isn't straight. Bouguer measured a mountain pulling a plumb line on Chimborazo in 1738. Maskelyne measured 11.6 seconds of deflection on Schiehallion in 1774, and Charles Hutton turned it into a density for the Earth. In India, Everest and Pratt found the Himalayas pulling less than they should. That led to the geoid, the lumpy surface a spirit level actually follows, and the GRACE satellites that mapped it from orbit.
Height depends on where you measure it from. A bridge between Laufenburg in Switzerland and Laufenburg in Germany came out 54 centimeters off because the two countries measured from different seas and a correction went the wrong way. In New Orleans, the Army Corps of Engineers found outfall canal protection built 1 to 2 feet lower than intended because of a height reference mix-up. The official US height datum is off by about half a meter, and its replacement has been delayed again and again. The video ends at LIGO, whose 4 kilometer arms had to be built straight rather than level and CERN's sloping collider tunnel, and back at Copa 201.
Subscribe to Trade Archive for more stories about the tools, machines and standards behind the trades: @tradearchive
Sources
Althouse, W., L. Hand, L. Jones, A. Lazzarini, and R. Weiss. "Precision Alignment of the LIGO 4 km Arms Using the Dual-Frequency Differential Global Positioning System." LIGO-P000006, 2001.
Dirección de Aeronáutica Civil, Panamá. Final Report, Copa Airlines Flight 201, Boeing 737-204, HP-1205CMP. 1992.
Moore, Wayne R. Foundations of Mechanical Accuracy. Bridgeport, CT: Moore Special Tool Co., 1970.
National Geodetic Survey. "New Datums." National Oceanic and Atmospheric Administration.
Petrie, W. M. Flinders. The Pyramids and Temples of Gizeh. London: Field and Tuer, 1883.
Smallwood, R. M. "Schiehallion, Maskelyne and the density of the Earth." History of Geo- and Space Sciences 9 (2018).
South Bend Lathe Works. How to Run a Lathe. South Bend, IN, 1934.
L.S. Starrett Company. "FAQ: 98 Series Levels."
US Army Corps of Engineers. Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System, Volume II: Geodetic Vertical and Water Level Datums. Interagency Performance Evaluation Task Force, 2006–2009.
Curry, Robert S., Jr. Erecting Means for Gyro Vertical Instruments. US Patent 2,408,411, filed August 16, 1944, and issued October 1, 1946.
Kanton Aargau. "Brücke Laufenburg: Korrektur nötig." Press release, December 24, 2003.