need to understan Thoreau

I'm working on my literature test. Can anyone help me to understand one of his expression?

That is an excerpt from Thoreau work "Life without principle"

Most men would feel insulted, if it were proposed to employ them in throwing stones over a wall, and then in throwing them back, merely that they might earn their wages. But many are no more worthily employed now. For instance: just after sunrise, one summer morning, I noticed one of my neighbors walking beside his team, which was slowly drawing a heavy hewn stone swung under the axle, surrounded by an atmosphere of industry,--his day's work begun,--his brow commenced to sweat,--a reproach to all sluggards and idlers,--pausing abreast the shoulders of his oxen, and half turning round with a flourish of his merciful whip, while they gained their length on him.

What did he mean by "they gained their length on him" ?

Thanks

i would infer that the oxen had moved ahead of the man, but not by much.

^Would it not be the reverse? He pauses when he is abreast their shoulders (i.e. front legs), and, urging them forward, waits til he is abreast the back legs before moving on.

At any rate, it just means he is hastening his team forward on the seemingly-important errand of moving the stone. The stone, if you read slightly further, winds up adorning someone's front yard or something, and so Thoreau sees the task of moving it as actually quite trivial.

I do believe Thoreau is referring back to the sluggards and idlers and is doing a bit of wordplay with length. In one respect he is inverting the a more common phrase "at length gaining" implying the sense that as the day passes those the driver scorn as lazy easily gain and pass him on their way. In a more poetic sense, as these same persons approach and pass, their lengths (shadows) gain and fall on the driver.