Frémont had a lot of problems keeping his chronometers running and maintaining their rates. Traveling over rough ground and in severe weather conditions made things very difficult. He had to pack around a 150X telescope to make observations of the jovian moons to check the rate of going or to re establish Greenwich time when they had stopped altogether?
Francis Drake didn't have any problems with his chronometers, because there weren't any back then. And there weren't any telescopes either, so nobody knew that Jupiter had moons. So he couldn't find his longitude, or even correct his solar declinations for his longitudinal position to get his latitudes accurately?
How easy it is today!
It also has a hack feature, so it is easy to hack
on to the time I get from my GPS receiver, which comes from
an atomic clock. I have to reset the watch twice a year at
the Daylight Savings Time changes. In between, I leave it
alone. I checked the other day, and in almost six months, it
has gained only 4 seconds!
And we also have a Time feed from an atomic clock right at hand when we glance at a cellphone, gps receiver, or computer!
I love mechanical devices, including timepieces with fine mechanical movements, but for keeping portable time, these take the cake.
"I think I may make bold to say, that there is neither any other Mechanical or Mathematical thing in the World that is more beautiful or curious in texture than this my watch or Timepiece for the Longitude......"--John Harrison, of H4.
The standard of accuracy required to qualify for the Board of Longitude's £20,000 prize finally claimed by John Harrison was 1/2 degree of longitude of a Great Circle on a 6 week voyage. 1/2 degree of longitude (about 34 geographical miles at the equator) is 2 minutes of time, or 3 seconds in 24 hours.
By comparison, my particular quartz wristwatch keeps time within less than 1 second a month.