Thus question refers to excrept from Mark Twain's novel "Roughing It".
The word "perceptible" would most effectively be explained as "capable of being perceived" and it originates from Latin word "percipere" which means to understand, to know something with one's senses.
In the paragraph 1, this word is used in a sentence " The snow lay so deep on the ground that there was no sign of a road perceptible...".
That basically means that, because of the snow, the road could not be seen, observed or recognized.
