问题：句子变成The currents of air that in the line of its course 学生还是不明白in the line of 该如何理解?

解答：你给出以上句子实际上省略“the walls of the ship direct ”补充完整后，你就很好理解了。补充完整后为下面句子。

The currents of air that the walls of ship direct in the line of its course.沿船航线向前的气流。

【1】the currents of air为名词“中心词”

【2】that the walls of ship in the line of its course.为关系代词that引导的定语从句修饰前面名词“the currents of air”关系代词that指代先行词“the currents of air”在定语从句作谓语动词direct的宾语。in the line of its course为定语从句地点状语。

特别备注：网友提供的句子：实际上是下面句子省略，as well as (the walls of the ship direct) in the line of its course是属于that定语从句里面内容。

【1】The currents of air that the walls of ship direct in the line of its course.沿船航线向前的气流。

【2】The currents of air that the walls of ship direct upwards.沿船体上升的气流。
把以上第1句和第2句用连词as well as 连接合并省略后就是下面英语句子

The currents of air that the walls of the ship direct upwards,as well as (the walls of the ship direct 省略)in the line of its course

参考译文：沿船体上升的气流和沿航线向前的气流。

把后面句子谓语全部补充完整后的语法分析如下：供网友参考

The currents of air that the walls of the ship direct upwards,as well as (the walls of the ship direct 省略)in the line of its course are enough to give the great bird with its immense wings sufficient sustenance and progress.

参考译文：沿船体上升的气流和沿航线向前的气流给这种巨翼大鸟以足够的浮力和推力.

备注(1): that the walls of the shop direct upward为定语从句修饰the currents of air. 关系的代词that指代the currents of air并在定语从句作谓语动词direct的宾语，the walls of the ship为定语从句主语。that为定语从句宾语，upwards为定语从句状语。

备注(2): as well as 后面省略与that定语从句相同的内容.

备注(3): in the line of +名词(sth) 沿着....的航线/路线

以上句子整体语法结构分析如下：供网友参考

The currents of air 【主语】//that the walls of the ship direct upwards,as well as(the walls of the ship direct  省略) in the line of its course【定语从句】//,are 【系动词】//enough【表语】// to give the great bird with its immense wings sufficient sustenance and progress【结果状语】.
参考译文：沿船体上升的气流和沿航线向前的气流给这种巨翼大鸟以足够的浮力和推力。