I read the following sentence
"They seemed like a double drift of stars, streaming through space."
In this passage:
"...Then this blood began to change too. Instead of a continuous liquid stream, Maskull perceived that it was composed of a million individual points. The red color had been as illusion caused by the rapid motions of the points; he now saw clearly that they resembled minute suns in their scintillating brightness. They seemed like a double drift of stars, streaming through space. One drift was traveling toward a fixed point in the center, while the other was moving away from it. He recognized the former as the veins of the beast, the latter as the arteries, and the fixed point as the heart."
What does "double drift of stars" mean here?