Cm hg At least the inner scale. 71.5 cm hg would be a bit over 28 in hg. Another scale may be hecto pascals.

I had to research this. Thought I had it figured out. But, then I realized I was mixing in/hg and meters altitude. Doesn't work. So, after googling a bit, I found that this is a Goulier pocket barometer. It can be used to find altitude difference between 2 points based on barometric pressure change observed. A little different from our modern pressure compensated altimeters.

If the inner scale is cm of mercury, and 2.5 cm = 1 inch, then a change of 2.5 cm must be about 1000' altitude, or 300 meters. So the outside scale must be meters.

1700m, or 5600'. So either it is broken or there is a very very warm front with extremely low pressure sitting over Paris.

170 meters, which would make sense if it is reading 28" of mercury (71.5 cm). Not that I would expect it to be accurate after all these years of not being calibrated.

The elevation of Paris is 35 meters, which means it's only 140 meters off. But who knows what the pressure altitude was that day...