PERSI (PERiodical Source Index) - "the largest subject index to genealogical and historical periodical articles in the world" - is available via Ancestry and at various libraries.
ILEJ, the Internet Library of Early Journals, is a joint project by the Universities of Birmingham, Leeds, Manchester and Oxford, conducted under the auspices of the eLib (Electronic Libraries) Programme. It has digitised substantial runs of 18th and 19th Century journals, and these images are now available on the Internet together with their associated bibliographic data.
The following titles are now available through the ILEJ service:- Blackwood's Edinburgh Magazine: Vol 53 - Vol 72 (1843 - 1852), Gentleman's Magazine: Vol 1 - Vol 20 (1731 - 1750), Notes and Queries: 1st, 2nd, 3rd and 4th series (1849 - 1869), Philosophical Transactions of the Royal Society: Vol 50 - Vol 67 (1757 - 1777).
Other titles will be added when available.
The entire back catalogue of Flight International magazine, going back to 1909, has been made available to browse, free, in searchable PDF format.