Galileo Galilei, Megafauna and Dinosaurs

Galileo's Two New Sciences, Day Two

This post is part of the series examining Galileo’s Two New Sciences,¹ his last work. Previous posts in this series are:

• How Two English-language Translators of Galileo Permanently Changed His Words

• A New Adaptation of Galileo's “Two New Sciences” for Modern Readers

• Galileo Galilei Supports Atomism, Part II

• Galileo Galilei on Falling Bodies

The dialogue of Day Two focuses on the resistance of bodies to breakage, according to their size. It turns out that the resistance of a body to breakage grows as the square of its outer dimensions, while the weight of the body grows as the cube of its outer dimensions. As a result, as a body with a given shape with given material is grown, there is a limit to the size of that body that can be supported, beyond which it will simply collapse from its own weight.

Galileo, after making the appropriate calculations and demonstrations, concludes that there is a limit to the size of machines that can be possibly constructed in a workshop, as well as to the size of possible animals in nature:

You now see how, from the things demonstrated thus far, there clearly follows the impossibility (not only for art, but for nature herself) of increasing machines to immense size. Thus it is impossible to build enormous ships, palaces, or temples, for which oars, masts, beamwork, iron chains, and in sum all parts shall hold together; nor could nature make trees of immeasurable size, because their branches would eventually fail of their own weight: and likewise it would be impossible to fashion skeletons for men, horses, or other animals which could exist and carry out their functions proportionably when such animals were increased to immense height — unless the bones were made of much harder and more resistant material than the usual, or were deformed by disproportionate thickening, so that the shape and appearance of the animal would become monstrously gross. Perhaps this was noticed by our very alert poet when, in describing a huge giant, he said:  

His height is quite beyond comparison,
So immeasurably gross is he all over. [p.127, my emphasis]

The emphasized part of the quotation refers to the size of animals. For a given kind of animal, there is a limit to how big it can be. He illustrates this by drawing the difference in bone size if the length of that bone is multiplied by three:

Here is his summary:

To give one short example of what I mean, I once drew the shape of a bone, lengthened only three times, and then thickened in such proportion that it could function in its large animal relatively as the smaller bone serves the smaller animal; here are the pictures. You see how disproportionate the shape becomes in the enlarged bone. From this it is manifest that if one wished to maintain in an enormous giant those proportions of members that exist in an ordinary man, it would be necessary either to find much harder and more resistant material to form his bones, or else to allow his robustness to be proportionately weaker than in men of average stature; otherwise, growing to unreasonable height, he would be seen crushed by his own weight and fallen. [pp.127-128]

Reading these passages of Galileo’s makes one wonder about the giant animals that have existed in prehistory.

For example, the now extinct megafauna of the Pleistocene era were often double the weight of the corresponding animals of today. The steppe mammoth of Siberia was double the weight of today’s African elephant. The Irish elk, which lived all over northern Eurasia, had antlers double the width of those of today’s North American moose, whose Eurasian counterpart is called an elk. And the list goes on, with giant bears, tigers, lions, sloths, etc. in the northern hemisphere, and giant marsupials, some carnivorous, in Australia.

But the size of the Pleistocenian megafauna is nothing in comparison to the size of the largest of the dinosaurs of the Jurassic and Cretaceous periods, with some estimated in the order of 100 tonnes or more. The scientists who first encountered these giant skeletons in the nineteenth century concluded that these creatures must have lived in swampy areas. However, it turned out that they did not live in swamps, but walked on land. Given what Galileo wrote above, how could they possibly have walked the earth?

I hope to return to these questions, once I have written about gravity.

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1. Galileo Galilei. Two New Sciences, Including Centers of Gravity & Force of Percussion. Translated by Stillman Drake. University of Wisconsin Press, 1974.