The Storms of Venus

I won't reprint the letter here, but a Zelazny fan whose first language is not English asked for some clarification regarding this exchange from "The Doors of His Face, the Lamps of His Mouth"

"You're from the Midwest, aren't you?"
"Yeah."
"Get bad storms out there?"
"Sometimes."
"Try to think of the worst one you were ever in. Got a slide rule handy?"
"Right here."
"Then put a one under it, imagine a zero or two following after, and multiply the thing out."
"I can't imagine the zeros."
"Then retain the multiplicand—that's all you can do."

It's a bit tricky to decipher because it involves both the archaic instrument of a slide rule and the word "multiplicand", which itself is hardly in common use.

I think the slide rule is something of a distraction. Davits is basically saying that you want to imagine a large Midwestern storm (which would be the multiplicand, the item which is multiplied) and the one followed by a one or two zeroes in the multiplier.

A one followed by one zero would be ten and a one followed by two zeroes is one hundred. Davits is telling Mike that the tropical storms on Venus are ten to one hundred times more powerful in the American Midwest.