We had, however, gone on but a short distance when a thick mist came sweeping along from the eastward, completely shrouding the whole country, as well as the sun above our heads, so that we had no object by which to direct our course. (In the Wilds of Florida by W.H.G. Kingston）
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。
We had, however, gone on but a short distance／（接続詞の前）when a thick mist came sweeping along from the eastward,／（コンマの後）completely shrouding the whole country,／（コンマの後）as well as the sun above our heads,／（接続詞の前）so that we had no object／（前置詞の前）by which to direct our course.
語句のかたまりを前から訳していきます。
We had, however, gone on but a short distance
しかし、私たちはほんの少ししか進めなかった
when a thick mist came sweeping along from the eastward,
そのとき、濃い霧が東方から押し寄せた
completely shrouding the whole country,
あたり一帯を完全に隠した
as well as the sun above our heads,
頭上の太陽も
so that we had no object
そのため、私たちには目標がなくなった
by which to direct our course.
進路を示してくれる
We had, however, gone on but a short distance when a thick mist came sweeping along from the eastward, completely shrouding the whole country, as well as the sun above our heads, so that we had no object by which to direct our course.
completely shrouding the whole country→ 分詞構文です。ing形の前に completely がついているので、少しわかりにくくなっています。
object by which to direct our course→ 普通の関係代名詞の文に書き換えると、object by which we can direct our course となります。
しかし、私たちがほんの少し進んだところで、濃い霧が東の方から押し寄せ、あたり一帯も頭上の太陽も完全に隠してしまった。そのため、私たちには進路を示してくれる目標がなくなった。
今回の課題文は、最初に短い「骨格」（中心部分）が置かれ、接続詞と分詞構文で詳しい状況説明が記されています。
We had, however, gone on but a short distance（ここまでが「骨格」）、when a thick mist came sweeping along from the eastward, （接続詞 when で「そのとき〜」）completely shrouding the whole country, as well as the sun above our heads,（分詞構文で補足説明）so that we had no object by which to direct our course. （接続詞 so that で「そのため〜」）
