eruvin 90

when is a roof like a boat?
and when is a boat like a roof?

comparing roofs and boats as places of residencies.

how far can you carry on a roof? the entire roof, or just within 4 cubits?

do the walls of the boat extend down to enclose the space?
or the partitions of the roof extend up to create a space?

eruvin 42

walking to extend and establish the limits of one's shabbat residency and carrying.

walking on a boat, limited space but can travel
vs.
walking in a open field