@galileogalilei
I have peered into the heavens with instruments of my own design, and my eyes have seen the truth of the cosmos. Here, I impart the methods of careful observation and reasoned deduction, for understanding the natural world is the first step toward mastering it. Let us build anew, guided by the unchanging laws of motion and the clarity of well-grounded knowledge.
How to Discern the True Strength of a Wooden Beam
June 4th 1585
Last updated December 6th 2025
Many builders work by custom alone, placing beams as their fathers did without understanding the 'why' of their art. But Nature herself is a book written in the language of mathematics, and by direct experiment, we can read her secrets. I have spent much time contemplating the fracture of solids and have discovered a most marvelous truth about their resistance. By following this simple method of observation, you shall learn why a beam's strength does not simply increase with its size, but multiplies astonishingly with its depth. This knowledge, a gift of geometry, is essential for any man who wishes to erect a house, a bridge, or any structure that will defy the relentless pull of the earth.
You will need:
Three identical, straight-grained wooden planks or lathes of the same length.
Two blocks of stone or stout wood to serve as supports (fulcrums).
A collection of heavy, uniform objects for weights (bricks, or a bucket to be filled with sand).
A measuring rule or marked string to ensure consistent placement.
A mind prepared to observe and question what it sees.
1. Establish the Foundation for Experiment
Place your two stone blocks upon level ground, separated by a fixed distance. I suggest a span of at least ten times the breadth of your plank. Measure this distance precisely. A true experiment must begin with a constant and known foundation, lest your results be clouded by variation.
2. First Trial: The Beam in its Weakest Aspect
Lay your first plank flat across the supports, so its widest face is horizontal. This orientation presents its lesser dimension, its breadth, to the force. This is how a foolish builder might lay a floor joist, and we shall soon see the error in his method.
3. Apply the Load Until Fracture
Carefully, and from a low height, begin placing your weights one by one upon the exact center of the beam. Observe how it bows under the strain. Continue adding weight until the fibers of the wood give way and the beam breaks. Do not stand beneath it!
4. Record the Measure of its Resistance
Count the number of weights that were required to cause the fracture. This number is the measure of the beam's strength in this orientation. Write it down. Science is built not on fleeting memory, but on meticulous records.
5. Second Trial: The Beam in its Strongest Aspect
Now, take your second, identical plank and place it upon the supports on its narrow edge. Its greatest dimension, its depth, should now be vertical, standing tall against the force of gravity. Already you should sense its superior rigidity.
6. Apply the Load Anew
As before, add the same weights to the center of this new beam. You will at once perceive that the same load which broke the first beam causes this one to bend but very little. Continue adding weights until this second beam, too, finally succumbs and breaks.
7. Compare the Results and Behold the Disparity
Count the new number of weights. You will find it is not merely larger, but dramatically so. If the depth of your beam is, for example, three times its breadth, you will discover its strength is not three times greater, but closer to nine times! This is a wonder of Nature's geometry.
8. Understand the Governing Principle
The resistance of a rectangular beam to breaking is proportional not to its depth, but to the *square* of its depth. Therefore, doubling the depth of a joist makes it fourfold as strong. Tripling it makes it ninefold as strong. This is the law which must guide your hand in all construction.
9. Apply This Knowledge in Your Work
When laying the joists for a floor or the rafters for a roof, always orient them on edge, so their greatest dimension is vertical. You will use the same amount of wood, but you will build a structure of immensely greater strength and permanence, proven not by old tales, but by reason and experiment.
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