A better, more dark angle:
The girl is a ragpicker, who stumbles upon the post card, which features beautiful imagery of a far away place she could never hope to go to, along with the orange in a parcel addressed to a little sickly but rich girl her own age, who lives down the lane.
The postcard is a reply from her father, who is sending her the exotic fruit (the orange) along with his reply, as he is overseas on business. The subject matter of the postcard is about how he wishes he could help her more in seeing other children her own age, but he is afraid of her getting sick by leaving the house, and wishes for her to remain in the care of her stuardess/governess until his return.
The ragpicker girl wonders about meeting this mysterious girl, and is resolute in returning her letter to her, but must elude the stuardess/governess to do so. (Being a rag picker girl, she is considered street filth, unfit for association with higher society.) The two meet, and become fast friends, each having something the other dreams of. (The rag picker girl can go ANYWHERE in the city, and be completely ignored-- and thus has complete freedom; the sickly rich girl has money, reliable food and housing, and "good breeding". (This IS steampunk setting, after all.) They both have a natural dislike for the stuardess/governess.
I think that's a better story that could be done with those elements. ;P
The most efficient thermal engine is the
stirling engine. It is right next to the ideal maximum that a heat engine could ever possibly be due to the
carnot efficiency limitation.Many stirling engine designs look VERY steampunky.