I was looking at the search for "Taylor Sling" on eBay and after seeing some gawd awful shoes that women are supposed to wear, mixed in were many nice Taylor units.
Some were used, some said used but obviously were still in the packaging. Ranged from $24 to way up around $80, with or without shipping.
I have a couple and are super nice units, made back when the thermometers were mercury, had fine engraved markings and such.
I don't know about the Belfort, but looks like a cool instrument and would not endanger anyone around you or the dog if he got too interested in the whirling thing his master was holding.
If you look at the charts and graphs to figure out humidity, it comes within a few % of the Relative Humidity for those values, especially if you use the chart with lines on it, rather than just extrapolating the numbers.
I'm wondering unless you use a Vaisala three salts humidity standard how close you'll get to the single digit accuracy, say nothing of the tenths of a point (and what difference can that be in the long run?) so maybe a sling is close enough.
I'm thinking that the slings were used back in the day when meteorologists were burly fellows who braved even weather that made the Post Office close, standing in the elements twirling their psychrometers not once but three times to get their best readings and then dutifully recording them.
As an aside, for those who haven't read The Children's Blizzard of 1888, there are some descriptions of the weather observer in St. Paul, MN or thereabouts doing his 4 x a day observations with an obsessive attention to detail and telegraphing the numbers on to Washington. There were some other descriptions of the lax observations (the original dry-labbing of data?) by other observes further west and up towards Canada.
Considering how long ago those observations were being made and what the guy could deduce about the coming storm from the data that was flowing across the telegraph, I was amazed at how intuitive he was.