Google Earth and Old Maps?!? Be still my heart!
Google Earth in 4D by ZDNet's Garett Rogers -- Google skipped right past the third dimension and landed directly in the fourth (time) by offering historical maps on Google Earth. Now you can travel back in time — for example, I am looking at the globe of 1790. Don't expect detailed high resolution photography from days gone by, but it's still interesting to see [...]
So, in order to make these fantastic archival maps transparent, look at the image below. If you cannot read it, you have to select the folder (highlight it) in the Rumsey collection under the Featured Content and then up in the Places section you'll see the Transparency slider.