How could that be observed?

It wasn't at that time. All we know is that average body temperature has been decreasing since the mid 1800s when we first started measuring it.

Just throwing an idea out there, mercury or alcohol thermometers were often used.

These would be far less pure in to 1800s. That would effect the accuracy of readings.

They were also hand made back then too.

If the theory of body temp being higher is predicated upon averages, it could skew. I wonder if such studies took this into account?