The letter, entitled “The Antideluvian Bones at the American Museum,” is an example of a scientifically-trained individual trying to accommodate the accumulating fossil evidence from North America within a biblical framework. After describing some of the fossil creatures (which he assures the reader are real and related to modern animals), he considers how such large creatures might have existed:
“We confess that the existence of those gigantic and antideluvian bones encounter in our minds no objection nor difficulty, but that of explaining by what law of nature a land animal could have existed and grown to the size of sixty feet in length and twenty five feet in heighth.
Those animals which are under our observation, and man himself, are subject to a law that generally, and with few exceptions, detain within certain limits their size and growth; and that is unquestionably the atmospheric pressure externally, and that which underbalances it in the organs of respiration.
This is the power which regulates circulation, and without it the transport of the matter necessary to compose and extend the body and limbs to a certain proportion. Hence we know that cetaceous animals of an extraordinary size can exist in abundance under the double pressure of the ocean and the atmosphere.
If the size, therefore, of the human race, and of that brute creation, originates from the pressure of about 2220 lbs. weight upon each square foot surface, the size of the Mastodon, or of any other mammoth animal, must have required an atmosphere three times heavier than it is at present. By what cause this change has taken place in the elementary orbit which surrounds our planet, it is beyond our power and philosophy to explain, unless we say that at the antideluvian period, and when the human race were giants, the waters above had not yet been separated and completely thrown down on the surface of the earth. But this theory would not comport with the prosperous condition of the human race before the deluge, when they were promised the long life of 120 years, and to be blessed with all the fruits of the earth, when for their corruptions at last they deserved to be exterminated by the flood.—(Gen. 4, 5, 6.
We would rather admit, between the orbits of Mars and Jupiter, the presence of a planet which has since been split by a comet into four parts, viz: Pallas, Juno, Vesta and Ceres, which have been scattered about, and so much diminished the weight of the atmosphere, that no longer giants or mastedons or mammoths are to be seen. FELIX PASCALIS.”
I would be very surprised if Pascalis was the first to propose that the pre-Flood earth had a higher atmospheric pressure. The idea is certainly still alive today: differences in the atmosphere of the pre-Flood earth (higher oxygen content, higher barometric pressure, etc.) are commonly hypothesized by Young Earth Creationists to explain the larger size of pre-Flood plants, animals, and humans, as well as the longer lifespans of pre-Flood humans discussed in Genesis.
Pascalis’ letter is interesting because it was an attempt to construct a systemic, natural explanation for both why both pre-Flood animals (of which fossil evidence was accumulating) and humans could have been be bigger (i.e., giants). I don’t understand how the asteroids (interpreted as fragments of a planet) would have diminished earth’s atmosphere or how that fits in with the Flood – I’m sure there’s more to that story. The four asteroids Pascalis mentions were first discovered in the early 1800s. The presence of the asteroid belt between Mars and Jupiter was not identified until later.
Pascalis' letter is a change from just a few decades earlier. Prior to the 1800s, the kind of direct evidence that Pascalis considered (fossil bones) was routinely interpreted as the remains of giant humans (see this post about Cotton Mather). In the absence of a concept of extinction, such an interpretation was logical: what else could the bones be but those of giants, since no other unknown creatures are mentioned in the Bible? Ironically, once mastodon bones were recognized for what they were, the actual physical remains that seemed to be proof of the existence of giant humans were no longer directly relevant. But wait, yes they were -- as shown by Pascalis’ letter, the impressive size of the mastodon bones could be used to build an argument for why pre-Flood humans (for which there then was no direct evidence) could, like the mastodon, also be large.