In a few days Captain Norton arrived with the ponies, and set my uncle’s mind at rest by reporting that the Indians had retired westward, and that it was supposed they would before long finally take their departure, according to the treaty they had entered into, and cross the Mississippi. (In the Wilds of Florida by W.H.G. Kingston）注：pony = ポニー（小型種の馬）
動画の後半部分でリスニング用音声が流れます。
英文を前から読んでいくために、語句のかたまりを作ります。こんな具合に課題文をスラッシュ（斜め線）で区切ります（スラッシュリーディング）。語句のかたまりを意識すると、リスニングで英語の意味をつかむのが楽になります。
In a few days Captain Norton arrived with the ponies,／（接続詞の前）and set my uncle’s mind at rest／（前置詞の前）by reporting that the Indians had retired westward,／（接続詞の前）and that it was supposed they would before long finally take their departure,／（コンマの後）according to the treaty they had entered into,／（接続詞の前）and cross the Mississippi.
語句のかたまりを前から訳していきます。
In a few days Captain Norton arrived with the ponies,
数日後、ノートン大尉はポニーを連れて到着した
and set my uncle’s mind at rest
そして、私のおじを安心させた
by reporting that the Indians had retired westward,
先住民が西へ退却したと報告することで
and that it was supposed they would before long finally take their departure,
そして、まもなく彼らは最終的に引き上げることになっている
according to the treaty they had entered into,
彼らが結んだ条約に従って
and cross the Mississippi.
そしてミシシッピ川を渡る
In a few days Captain Norton arrived with the ponies, and set my uncle’s mind at rest by reporting that the Indians had retired westward, and that it was supposed they would before long finally take their departure, according to the treaty they had entered into, and cross the Mississippi.
set my uncle’s mind at rest→ set one’s mind at rest = 〜を安心させる
it was supposed→ it は形式主語です。意味上の主語は they would〜（they の前の that が省略されています）
数日後、ノートン大尉はポニーを連れて到着した。そして、先住民が西へ退却したこと、そして彼らが結んだ条約に従って、まもなく彼らは最終的に引き上げ、ミシシッピ川を渡ることになっていると報告することで、私のおじを安心させた。
In a few days Captain Norton arrived with the ponies, and set my uncle’s mind at rest by reporting that the Indians had retired westward, and that it was supposed they would before long finally take their departure, according to the treaty they had entered into, and cross the Mississippi.
by reporting（報告することによって）という語句がありますが、何を「報告」するのでしょうか？ reporting の直後にある that節（that the Indians had retired westward）が１つですが、もう１つあります。and that it was supposed〜 の that節も、報告の内容になっています。
they woulld につながる動詞も２つあります。１つは take で、もう１つは cross です。
ペアになっている語句の２つ目を見逃すと、意味がとれなくなるので注意しましょう。
