Fleeting notes are similar to the sorts of things one would have traditionally kept in a waste book.

Francesco Sacchini recommended the use of two notebooks:

“Not unlike attentive merchants... [who] keep two books, one small, the other large: the first you would call adversaria or a daybook (ephemerides), the second an account book (calendarium) and ledger (codex).” —Francesco Sacchini "Chapter 13". De ratione libros cum profectu legendi libellus. Wurzburg. p. 91. (1614).

(See also Blair, Ann M. (2004). "Note taking as an art of transmission". Critical Inquiry. 31 (1): 91. doi:10.1086/427303.)

The root word ephemeral in this context is highly suggestive of the use and function of fleeting notes.

The Latin word "ephemerides" can also be translated as "newspaper", useful for only a short period of time.

Recall also that in a general sense Cicero contrasted the short-lived memoranda of the merchant with the more carefully kept account book designed as a permanent record.

Reference: Cicero (1930). Pro Quinto Roscio comoedo oratio,"The Speeches". Translated by Freese, John Henry. Cambridge, Massachusetts. pp. 278–81.

Cicero (1930). Pro Quinto Roscio comoedo oratio,"The Speeches". Translated by Freese, John Henry. Cambridge, Massachusetts. pp. 278–81.

(Not sure if I had this in my notes already from other reading, but adding again just in case.)

Dickens evidently drew this idea from a note written earlier in his Book of Memoranda: "Gibbon's Decline and Fall. The two characters, one reporting to the other as he reads. Both getting confused as to whether it is not all going on now!" (21). Boffin and Wegg are reading Edward Gibbon's The History of the Decline and Fall of the Roman Empire, which was published in six volumes between 1776 and 1788. Dickens had an 1825 eight-volume edition.