The typographic measuring system 17页

章节如上，17页，在说到didot 的时候，有两个句子，没弄懂

原文如下：

it is a system correlated with french foot of 30 cm. The line gauge corresponds to the french foot. it is 30 cm long and measure 798 typographic points.

actual type height of 62% 2/3 points was not uniformly fixed until 1898.

这几句有高人知道怎么翻么？什么意思？