How to find biography (dates of birth/death, etc) of prominent 18th Century English person? Hugh Clark is the author of "An Introduction to Heraldry", but I could not find anything about his biography and even his dates of birth and death. How can I learn more about an 18th century author such as this?

The British Library have a number of works by him listed in their catalogue and record him as active from 1775 to 1784.

There is also an entry in the Oxford Dictionary of National Biography which may relate to him and which gives dates of 1745 to 1822 though you need a subscription (or access to the printed version) to see the whole entry.