Tuesday, July 29, 2008

How Pyramids Work-2

How Were the Pyramids Built?
Pyramid construction is a continuously debated topic. There are no existing records of building plans or discussions of construction methods, so no one knows exactly what happened. Of course, archaeologists and engineers have plenty of ideas -- some sound far-fetched and others seem more reasonable. We'll use the Giza pyramids as an example because we have the most information on them.
Let's break the task of building the pyramids into components:
Surveying and excavation: choosing a suitable site, orienting it and preparing the foundation
Obtaining building materials: quarrying rocks or making huge stones
Transporting building materials: transporting from the quarrying site to the pyramid
Workforce logistics: finding skilled workers, feeding them and housing them
Surveying and Excavation
Egyptian builders probably made plans and models of the pyramid. The projects were overseen by the pharaoh's master builder, or vizier.

The collapse of the Meidum pyramid and the shift of the Bent Pyramid taught builders that foundations were important. Once engineers found a suitable site with a good foundation, they had to lay out the site.
Pyramid sides always ran parallel to the north-south and east-west axes. The builders didn't have compasses, and there was no North Star at the time (the Earth's rotation wobbles like a top, and the position of true north in the sky changes over a 40,000-year cycle). So, they used the movements of circumpolar stars or the sun to figure out true north. Using sighting rods and circles, they could trace arcs of the rising and setting stars or the sun's shadow, measure the angles to the ends of the arc, and calculate true north. Once they established that, they could find the other directions with lines and right angles.
The ancient Egyptians used "cubits" (the length from the tip of your middle finger to your elbow) and "hands" (the width of your hand with the thumb on the side) for measurements. They dug post holes at regular intervals (10 cubits) along the base outline and laid out the site in a grid.
Then, laborers excavated and leveled the foundation. No one is sure of the exact method, but they were extremely exact -- the base of Khufu's pyramid is level to 2 centimeters (less than an inch).
There are two main theories about leveling methods:

1) Workers poured water into the excavated site and leveled all material above the waterline. Then they lowered the water level and removed more material, continuing the process until the foundation was level.

2) The builders installed posts at regular intervals. A line, leveled with plumb bobs, was pulled taut across the posts at a reference mark to ensure alignment. Then they could excavate the foundation down to the reference marks.