Re: Age Earth was thought to be

Some of the more famous calculations include:

Archbihop Usher:  Used the Bible to calculate an age of ~6000 years.  He 
went so far as to specify time and date in the year 4004 BCE (before the 
common era - equivalent to BC, but without the direct reference to Christ). 

Hutton: An early uniformitarian saw "no vesitge of a beginning, no prospect 
of an end" - in other words, the earth had always been and would continue 
onward with the same processes.  For a really wonderful account of Hutton's 
views of geologic time, read "Time's Arrow, Time's Cycle" by Steven Jay 
Gould.

Lord Kelvin: Used cooling rate to establish an age of ~100 million years, 
later revised that down, but he didn't know about heating because of 
radioactive decay.

A very nice history of views on the age of the earth can be found at the 
talk origins FAQ:
 http://www.talkorigins.org/faqs/geohist.html

Other pages on talkorigins.org have information about radioactive dating 
and the history of that technique.  There are also explanation of the 
problems with commonly misused techniques (commonly cited by young-earth 
creationists in spite of abundant and extensive refutations) for dating the 
earth.

David Smith
Geology and Environmental Science
La Salle University, Philadelphia, PA