A doubt in Special Relativity

I was reading a introductory text on Special Relativity. It said somthing about Space time which I couldn't logically suffice.
For space time interval, we have the relationaship :

s^2 = (x^2)+(y^2)+(z^2)+(ict)^2.

If this is the case, then, if something would have just stood where it is, and would have travelled only through time, the space time interval in travells would have shortened (as i^2=(-1) ) with passage of time. Should it not have increased with passing time, logically?

Another thing - How in the holy heaven did Einstein ever imagine that speed of light is independent? He certainly didn;t know about michaelson experiment when he came to this conclusion. Was it by Maxwell Equations (that speed of light depends only on permittivity & permeablity of medium)? If it was, then why didn't somone see that very obvious thing for decades?