Hi, Please Help To Explain The Following Question

How To Calculate RH From Celsius Which Show In Mercury?

I Used 2 Mercury:
~First Mercury Covered With Damp Cloth Temperature Show is 21'C
~While, Another Mercury Show Room Temperature Which is 26'C.

How To Calculate The Relative Humidity% (RH) from temperature shows above?

What is The Formula To Calculate? Please Advice. Thanks...