Treatise on Natural Philosophy

Treatise on Natural Philosophy was an 1867 text book by William Thomson (later Lord Kelvin) and Peter Guthrie Tait, published by Oxford University Press.

The Treatise was often referred to as ${\displaystyle T}$ and ${\displaystyle T^{1}}$, as explained by Alexander Macfarlane:^[1]: 43

Maxwell had facetiously referred to Thomson as ${\displaystyle T}$ and Tait as ${\displaystyle T^{1}}$. Hence the Treatise on Natural Philosophy came to be commonly referred to as ${\displaystyle T}$ and ${\displaystyle T^{1}}$ in conversation with mathematicians.