Referencing increasing rows vertically.
 

I'm not even sure how to say what I need so I'll ask via example. I have 12
columns in Sheet1 labeled Jan-Dec on row 1. On Sheet2, down column A,
beginning on row 1, I want to reference the 12 months vertically. In other
words, cell A1 equals "Jan," cell A2 equals "Feb" etc. The simple formula I
use in cell A1 of Sheet2 is =Sheet1!A1. If I click, hold and drag down the
bottom-right corner of cell A1 to replicate the formula downward, the formula
changes by incrementing the row reference. I want it to increment the
column. In other words, I want cell A2 to be =Sheet1!B1, etc.
How do I do that?