Complex sentence: By now, it had become clear that these islands were desolate and quite useless and certainly not the Terra Australis, on his return Kerguelen-Trémarec was sent to prison.
Split into simpler sentences: By now, it had become clear that these islands were desolate and quite useless and certainly not the Terra Australis. On his return, Kerguelen-Trémarec was sent to prison.