This specimen is an excerpt from A New System of Castle Building that appears in The Student: or the Oxford and Cambridge Miscellany Monthly - Volume 2 (a compendium magazine published by, you guessed it, Oxford and Cambridge Universities).
Regarding the dashes:
What we have here are visual cues in the text for purposes of reading this essay out loud (I suppose the breaks can also be emulated in your head silently reading this prose). My guess is the use of these dashes in poetry was fairly common since raconteuring was commonplace in the 18th century and there are more examples of this that a appear in poetry later in the same publication (see further examples below). The complete digital version of this edition of The Student: or the Oxford and Cambridge Miscellany Monthly - Volume 2 can be found free here. In this sample, which is a spectacular specimen, the length of the dash represents the length the reader should pause and emphasizes the break in the flow of reading out loud.
The dash marks some kind of pause -- in reading aloud, one might accord it the duration of a comma, unless it is a long em-dash requiring a lengthier break. Dashes can mark the caesura of the poetic line. But let's look at a couple of examples to see just how complex the phenomenon can get.
-What is the effect of a dash in poetry? What are some examples?
Another interesting aspect that caught my attention while looking through digital version of The Student, was if we back up a page to the beginning of this essay, we'll find that it appears under a section of the magazine titled “Calliope”, which is a pretty obvious reference to the muse who presides over eloquence and epic poetry and a strong indication that what follows, is going to be some poetry or prose:
Prose in its simplicity and loosely defined structure is broadly adaptable to spoken dialogue
- Prose (Wikipedia entry)
Prose is the form of written language that is not organised according to formal patterns of verse. It may have some sort of rhythm and some devices of repetition and balance, but these are not governed by regularly sustained formal arrangement. The significant unit is the sentence, not the line. Hence it is represented without line breaks in writing.
- Prose vs. Poetry Definition
This leads me to believe that although Castle Building is technically classified as an essay in this publication, there is evidence that points to the author's intention of his writing to be presented as prose. Furthermore, the author is indeed Christopher Smart (submitted under the pseudonym Chimaericus Cantabrigiensis) who is most notably a poet. Regarding his writings as Chimaericus in particular :
Smart's experimentation with the comic disjunction between a lofty acedemic discourse and the everyday flow of the vernacular of the street or coffeehouse...
- Christopher Smart and Satire: 'Mary Midnight' and the Midwife (p. 86)
My initial thought was that this example was an unorthodox use of em-dashes to visually block “rivers” created by justified text. However, the earliest published style guide I could find that mentions a variety of em dashes that includes one large enough to fit the longest dashes in the sample, was in the 1911 edition of the Chicago Manual of Style Manual. Even then, is no specific mention of dash usage in poetry:
Here are some more poetry samples that utilize dashes from the same 1751 publication of The Student: or the Oxford and Cambridge Miscellany Monthly - Volume 2. Note: The use of dashes are not as prevalent in these samples as the authors may not have been as focused as Smart was on vernacular. Additionally, these samples are written in poetic verse using formatting (i.e; line breaks) to accomplish most of the pause effects and overall rhythm:
This is a specimen of print wherein the typesetter is carrying out the author's intention of employing typographical elements (i.e, dashes of varying length) as a tool to convey the cadence of the common street vernacular of the era primarily for purposes of performing this essay as prose in spoken word (read out loud).
The Student: Or, The Oxford and Cambridge Monthly Miscellany, Volume 2
Christopher Smart and Satire: 'Mary Midnight' and the Midwife
A Manual of Style, Volume 5; Volume 7
Christopher Smart - Wikipedia entry
Prose - Wikipedia entry
Calliope - Wikipedia entry
What is the effect of a dash in poetry?
Prose vs. Poetry Definition