BoingBoing alerted me, last Friday, to the free accessing (this month only) of Scientific American weeklies from 28 August 1845 to 25 December 1909. A weeks-between lookup gives 3357 potential issues, but I think eight of those did not see publication. Of the remaining 3349 (that's four more than claimed by Scientific American), I managed to find 3246 complete, readable issues at the site, some hidden behind missing or misdirecting links.
I have been an aficionado (and eventual collector) of the magazine since discovering this issue near the end of 1967. Having a substantial, freely accessible pdf-archive of the old weeklies was, for me, a little like finding money on the street. I spent all day Saturday downloading, as a result of which I woke up Sunday morning with a debilitating lower-back issue (it became extremely painful to maintain an upright position) from which I did not recover until yesterday! I spent all of last night (until about 4:30 this morning, more carefully minding my posture) completing the heist.