Goliath bird-eating spider, Theraphosa blondi.
Image: Snakecollector (Creative Commons Attribution 2.0 Generic license) [goliathise]
What is the world's heaviest spider? The mere suggestion of the two words -- "heaviest" and "spider" -- standing next to each other in a sentence is enough to strike terror into the minds and hearts of arachnophobes everywhere. After recovering from our initial terror, this raises the question: how would you even figure this out? Do you place giant, angry, spiders with huge fangs onto a scale and hope they sit still long enough for you to get an accurate mass before they chase you around the room?
This was the very question that Natural History Museum "bug" expert, George Beccaloni, was asked to solve when he was contacted by Guinness World Records Editor-in-Chief, Craig Glenday. Craig was following up reports of a possible rival to the currently recognised heaviest spider in the world, the female goliath bird-eating spider, Theraphosa blondi, from South America. Craig contacted George, the author of the book, Biggest Bugs Life-Size [Amazon UK; Amazon US], and asked if he could check the size of the Hercules baboon spider, Hysterocrates hercules, from Africa, since the Natural History Museum has the world's only known specimen.
This fun video shows you how this question was answered and also gives you a behind-the-scenes peek at the arachnid collections at the museum:
If you were listening carefully to the video, then you may have caught the name of the scientific method used to measure these spiders: Archimedes Principle. This principle is named after its discoverer, Archimedes of Syracuse, who stated that "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object." In short, the volume of displaced fluid is equal to the volume of the object itself.
Archimedes is recognised as being one of the greatest mathematicians of all time. Archimedes is probably best known for running through the streets of Syracuse, shouting "Eureka!" According to this legend, this occurred after Archimedes had suddenly realised how he could honour King Hiero II's request to determine whether his new crown was pure gold.
As the story goes, the king had provided pure gold to a goldsmith to make a crown. After he'd received the completed crown, the king was suspicious that it was not pure gold, that the goldsmith had mixed in some less valuable silver and kept an equivalent amount of gold for himself. So the king asked Archimedes to determine whether his suspicions were correct -- without damaging his crown (just like George and his spiders).
Archimedes noticed that the level of water in his bath tub rose when he got in, and realized he could use this principle to determine the volume of the king's crown. Because water is incompressible, Archimedes could calculate the volume of the crown by submerging it. Then, by dividing the weight of the crown by the volume of water displaced, he could determine whether the crown was pure gold. If less dense a lighter metal, like silver, (or another lighter metal) had been mixed into the gold, the crown would be less dense weigh less for the same unit volume than if it had been made with pure gold.
Excited by this discovery, Archimedes jumped out of his bath and, forgetting to dress, ran naked through the streets, crying "Eureka!"
Of course, the precise details of this legend are probably not true, but it leaves a lasting impression on people's minds, which explains why this story is still repeated to this very day.
Oh, and the king's crown? Archimedes proved that silver had been mixed into the gold. I have no idea what happened to the goldsmith, but I'd guess he didn't leave as many offspring in the world as he could have done had he not tried to cheat the king.
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Corrigenda [added 21 July 2011]: reader John Bey scolded me in email today regarding an error I made in this piece. In short, I misused the words "density", "volume" and "weight".
For those who were paying attention, density refers to the internal packing of the atoms that comprise an object: higher density means that the atoms are packed more tightly inside the object.
Volume is simply the measure of the space occupied by a substance. For example, gases expand to fill their container (but in doing so, they reduce their density).
Weight and mass are deceptively similar. Mass is a constant property of an object that depends upon the number and type of atoms that comprise it. Mass does not change for a particular object unless the numbers or types of atoms comprising it are changed.
Weight is more complex than mass, but it is related to it. Weight measures the gravitational attraction between two objects. This is dependent upon distance between those objects as well as their masses and densities -- closer objects have a stronger gravitational attraction, as do objects with higher densities. This is how an astronaut can be much heavier on Earth than on the International Space Station, despite having the same mass.