The 1900s

Some pre-1960 examples of "in the 1800s" at Google Books, (there are less than thirty entries all told) which I presume refer to the century:

"it would triple the former record set in New York during subway construction late in the 1800s" 1958

"The minor force behind output expansion in the 1800s became the major force in the 1900s." 1958

"Most breeds were established in the 1800s by dog fanciers, using a small number of founders that featured traits of particular interest" 1952

"Now, Sir, I would suggest that if that was true in the 1800s, it is probably no less true to-day," 1959

Only a few appear to definitely refer to the decade:

"Land disturbance began again in the 1800s and culminated in the 1880s" 1923

"It appears that a previous pastor of the same church back in the 1800s had a son, Woodrow Wilson, who grew up there" 1944

Though many are admittedly ambiguous without looking at the actual contexts. Going back further we find:

"The Lakota had migrated from Minnesota to the plains in the 1700s. Here they developed the classic plains culture. After the Civil War they fought against the United States to keep their lands but were concentrated on reservations in the West," 1846 (perhaps ambiguous)

In all countries having the social cleavages and the feudal survivals of England in the 1700s and early 1800s, the offenders against the criminal law come in the far greater proportion from what are known as the " lower classes," 1899

" Men and women of both classes flooded the colony in the 1600s and early 1700s and had an enormous impact on both the population of the colony and its laws. U.S.A. " 1895

But there are also examples where no doubt the decade is being referred to.

I imagine that this expression has long been used in both senses, except when talking of the century we're living in. For us oldies, much of our lives was lived in the twentieth century, when naturally the 1900s was used for the decade, but I'm not so convinced about the 1800s. As with much in language, it simply depends on context. And as soon as things like "early, mid, late" are added, it seems more likely that the century is being referred to.

Interestingly Ngram suggests that this expression wasn't much used before the mid twentieth
century, even for the 1700s and 1800s. Incidentally, the written out forms hardly register.

http://books.google.com/ngrams/graph?content=the+seventeen+hundreds%2Cthe1600s%2Cthe+1700s%2Cthe+1800s%2Cthe+1900s&case_insensitive=on&year_start=1600&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cthe%20seventeen%20hundreds%3B%2Cc0%3B.t1%3B%2Cthe%201700s%3B%2Cc0%3B.t1%3B%2Cthe%201800s%3B%2Cc0%3B.t4%3B%2Cthe%201900s%3B%2Cc0%3B%2Cs0%3B%3Bthe%201900s%3B%2Cc0%3B%3BThe%201900s%3B%2Cc0

I completely agree with you, Warsaw Will. Our writers at http://www.professional-essay-writing.com have investigated the case thoroughly. They do suggest the idea you prompted.

Not another one! I'm getting fed up with these. Reported.