One thing I didn't see in Salem, Massachusetts (I started an entry about going to Salem but haven't finished it because it's depressing) because I didn't know it was there but would have visited had I known about it is the Nathaniel Bowditch House.
Who, you are probably asking, is Nathaniel Bowditch?
Nathaniel Bowditch was a very important self-taught navigator who found some important way of determining one's location while at sea. His work The American Practical Navigator, published in 1802, is still in print (seriously--you can get it from Amazon) and is carried on all commissioned US Naval vessels.
That's not a very complete explanation, but the two sites I link to--both Wikipedia and some Salem history thing--give a more thorough, learned explanation than I could provide even if I cribbed from them extensively.
But I was able to give you a bare-bones answer in part because as I child I read, and earlier this summer I reread, the absolutely marvelous Newbery medal-winning Carry On, Mr. Bowditch by Jean Lee Latham. I reread it in part because back when Anonymous Blog Friend visited me, ABF and I visited the Flagship Niagara, which was very cool* but made both of us wonder why we have this fascination with maritime history. (Aside, of course, from the fact that Ioan Gruffudd, who plays Horatio Hornblower, is so HOT!--which actually still doesn't answer the question, because not everyone wants to watch even a hottie like Ioan portray an early 19th-century British sea captian.) And then I noticed Latham's book in the gift shop, and said, "Reading this in fourth grade or so probably has something to do with my interest in tall ships. Have you read it? It's really good."
Rereading it this summer, I still thought it was pretty good. If you want a quick intro to 18th-century navigation, try this! It's a fun little historical novel and unless you already know a lot about naval history, you'll be smarter when you finish it than you were when you started.
Bowditch grew up in Salem; I'd forgotten that connection until I saw the headstone of his first wife, Elizabeth, in the old graveyard. She died when she was only 18. I searched the Bowditch plot of Nathaniel's grave, but it wasn't there; turns out he's buried in Cambridge, which I visited the next day, but I didn't go looking for cemeteries while I was there--just Indian food and universities, all of which I found.
*One thing I just LOVE, in that "this horrifies, revolts and fascinates me" kind of way, is a video about the damage cannon fire does to a ship. When they were building the replica of the original brig, they also built an extra ship side, then took it out to the middle of nowhere, and fired cannons at it. I find the video so compelling because it makes me realize what I hadn't known before: I hadn't known about shrapnel. I mean, I knew there was this thing called shrapnel, but I didn't realize that when a cannon ball hit some great big old boat, it would cause the timber the ship was constructed from to splinter into sharp, jagged chunks of wood often bigger than baseball bats, which were hurled about with great force, and could do a lot of damage to human bodies in their path. Even an itty bitty piece of shrapnel--say, six inches long--could really freaking HURT if it went right through your lower abdomen or shoulder or face at 60 miles an hour.