Let's look at hands and fingers of her in this drawing.
Some fingers looks almost putting on the space bar.
Mrs. Longley might not be touch typist until then and later so, almost the first human who noted to public to use thumb to push or strike down the space bar, it is not recommended but as a thing must.
On thumb, at page7 bottom of
Elizabeth Margaret Vater Longley "Type-Writer Lessons for the Use of Teachers and Learners Adapted to Remington’s Perfected Type-Writers"(Cincinnati, 1882)Never strike the space-board with the fingers, but always with thumb.
Without caution like she informed, people might be apt to use other than thumbs.
And this can only happen on Remington(or Sholes&Glidden) QWERTY machine, not on Caligraph.
- There were almost only two choices they could take on keyboard typewriter having typebars, Remington or Caligraph.
- I'll write some later on an article http://bqaa.blogspot.jp/2014/03/it-s-apparent-getting-into-era-typing.html
On her book for Caligraph, there was no mention how to use thumb and read as follows:
At the bottom of page 7:Caligraph Lessons for the Use of Teachers and Learners Designed to Develop Accurate and Reliable Operators』(Cincinnati, 1882)Strike the space-bar with the little finger.
I've written things mentioned above, before Oct. 2010 or so at the latest, in Japanese.
But these weren't early.
Mrs. M. V. Longley: ``Writing Machines'', Proceedings of the First Annual International Congress of Shorthand Writers, pp.14-16 (August 31, 1882).
"the thumb will be in readiness to strike the space-bar"
- http://books.google.co.jp/books?id=q_HL6GQi5BEC&pg=PT290&dq=%22thumb+will+be+in+readiness+to+strike+the+space-bar%22 (2009)
- http://slashdot.jp/~yasuoka/journal/411996/ （2007, Aug. 11th)