“Definite portions of a manifoldness, distinguished by a mark or by a boundary, are called Quanta. Their comparison with regard to quantity is accomplished in the case of discrete magnitudes by counting, in the case of continuous magnitudes by measuring. Measure consists in the superposition of the magnitudes to be compared; it therefore requires a means of using one magnitude as the standard for another. In the absence of this, two magnitudes can only be compared when one is a part of the other; in which case also we can only determine the more or less and not the how much. The researches which can in this case be instituted about them form a general division of the science of magnitude in which magnitudes are regarded not as existing independently of position and not as expressible in terms of a unit, but as regions in a manifoldness.”

Book 1, Propositions 6 & 7, The Law of the Lever.
On the Equilibrium of Planes

Zero can never be a unit.
Definition 3. Part 1. The Elementary Conjunctions of Extensive Magnitudes. Ch. 1. Addition, Subtraction, Multiples and Fractions of Extensive Magnitudes. 1. Concepts and laws of calculation. Extension Theory Hermann Grassman, History of Mathematics (2000) Vol. 19 Tr. Lloyd C. Kannenberg, American Mathematical Society, London Mathematical Society
Ausdehnungslehre (1844)

Source: The Human Form: Sculpture, Prints, and Drawings, 1977, p. 10.

Footnote: The apparent advantage of the generality of this definition of number disappears as soon as we consider complex numbers. According to my view, on the other hand, the notion of the ratio between two numbers of the same kind can be clearly developed only after the introduction of irrational numbers.
Stetigkeit und irrationale Zahlen (1872)

1860s, Allow the humblest man an equal chance (1860)
Context: Look at the magnitude of this subject! One sixth of our population, in round numbers — not quite one sixth, and yet more than a seventh, — about one sixth of the whole population of the United States are slaves! The owners of these slaves consider them property. The effect upon the minds of the owners is that of property, and nothing else — it induces them to insist upon all that will favorably affect its value as property, to demand laws and institutions and a public policy that shall increase and secure its value, and make it durable, lasting and universal. The effect on the minds of the owners is to persuade them that there is no wrong in it. The slaveholder does not like to be considered a mean fellow, for holding that species of property, and hence he has to struggle within himself and sets about arguing himself into the belief that Slavery is right. The property influences his mind. [... ] Certain it is, that this two thousand million of dollars, invested in this species of property, all so concentrated that the mind can grasp it at once — this immense pecuniary interest, has its influence upon their minds.

Act II, scene ii
Love for Love (1695)
Variant: Ferdinand Mendez Pinto was but a type of thee, thou liar of the first magnitude.

Letter http://founders.archives.gov/documents/Adams/99-02-02-7261 to William Tudor, Jr., 20 November 1819. Partially quoted in Founding Brothers : The Revolutionary Generation (2000) by Joseph J. Ellis, p. 240
1810s
Context: I Shall not pause to consider whether my Opinion will be popular or unpopular with the Slave Holders, or Slave Traders, in the Northern the Middle, the Southern, or the Western, States—I respect all those who are necessarily subjected to this Evil.—But Negro Slavery is an evil of Colossal Magnitude. … I am therefore utterly averse to the admission of Slavery into the Missouri Territory, and heartily wish that every Constitutional measure may be adopted for the preservation of it.

Source: Sir William Herschel: His Life and Works (1880), Ch.4 "Life and Works" from a memoir, published (1817).
Context: It is evident that we cannot mean to affirm that the stars of the fifth, sixth, and seventh magnitudes are really smaller than those of the first, second, or third, and that we must ascribe the cause of the difference in the apparent magnitudes of the stars to a difference in their relative distances from us. On account of the great number of stars in each class, we must also allow that the stars of each succeeding magnitude, beginning with the first, are, one with another, further from us than those of the magnitude immediately preceding.

Atlas, on bearing the burden of maintaining the worlds, in Ch. 4
Space Chantey (1968)
Context: I tried to tell you, but words will not convey it. One has to be inside it to comprehend the magnitude. … It was the beginning. It's the only thing there is. But it was haphazard for so many aeons that it spooks me to think about it. There were always three or four maintaining it, but there was no one person strong enough to take it all over. "Somewhere there must be someone strong enough to take it all over," I said to myself in a direful moment, but the strongest person I could think of was myself. I've been doing it ever since. … By my attention I hold it all in being. Nothing exists unless it is perceived. If perception fails for a moment, then that thing fails forever. … I hate to be misjudged. They say that I bear it all on my shoulders, as though I were a stud or a balk. It's not on my great shoulders, it is amazing head on my great shoulders that maintains all.