1857 diagram of a "Vault Cover": "Illuminating Roofs and Roof Pavements". Another simple cross sections of a sidewalk basement roofed with cast-iron. In <glassian.org>.
"[T]he purpose the invention being to change the use of the space underneath sidewalks from coal-vaults to finished apartments capable of becoming a portion of the basement."
"I do not claim to be the first to enlarge a basement by taking into it the area-space, nor the first to incorporate the area-space into the basement by a glass covering; but I do set forth as my invention that my glass covering does not barricade the doorways of the building. ... The only actual basement extension which I found when I began my improvements was where the area-way alone was taken into the basement, not the vault beyond it, and this area was covered by a skylight, and this skylight was above the level of the street, and then the whole thing was cut off from the sidewalk by an iron railing. This was the state of the art as I found it."
"All other underground rooms existing at that period, those at the New York Sun buildings included, were simply vaults, and lighted from light-holes. All were apartments distinct from the basements, and none of them were able to add light to the basement, for they had not sufficient light for themselves."
"The first thoroughly-lighted vault ever constructed was made so by a sidewalk of my lights, laid by me for the New York Herald building in the year 1850. This enabled me, two years later, to get the opportunity of laying down an actual basement extension, taking in all the space under the sidewalk, and going two stories under ground; but that year, the following, and the next were required in order to perfect the work so as to secure the public confidence, which was accomplished only when, after repeated failures, I at length succeeded in making water-tight joints that would stand both summer and winter and concussions of every kind and constantly repeated."
"Drawing B is a sectional elevation of a building, where A is the area-space underneath the area-light A L. V S is the vault-space under the sidewalk S W, here formed of granite slabs. B is the basement. P S is the first or principal story. d s is the illuminating door-sill. r is the illuminating-rider under the door-sill; F, the "rising lip" of the area-light. G is the nosing of the area-light where it sets over the iron riser at the sidewalk."
"[T]he extension of basements under the street by means of an illuminating-roof pavement is my invention, for a roof-pavement in such a combination of glass with iron as to be fit for being walked upon. What I mean to say is that until I did it no area-way was ever covered, and no basement was ever extended under the street by glass and iron combined to form substantially a portion of the sidewalk itself."