Why does Wargrave put his confession in a bottle?

Why does Wargrave put his confession in a bottle? Why does Wargrave put his confession into a bottle? So, someone would discover the confession and his genius . Before discovering his body, why do the remaining guests believe Armstrong is dead?

Why does Wargrave put his confession into a bottle? So, someone would discover the confession and his genius . Before discovering his body, why do the remaining guests believe Armstrong is dead?