Throughout the rest of the 18th century and through most of the first half of the 19th century, ale was still distinguishable from beer almost exclusively on the amount of hops used in the brew. The Complete Dictionary of Arts and Science of 1773 defined the word ale as "a fermented liquor obtained from an infusion of malt and differing from beer in having less proportion of hops". On top of that, porter became distinctive in that it was darker and often more bitter than beer. This is echoed by several definitions of the age (I will not bore you with them) up until the 1830's with the development of a new type of pale ale produced with large amounts of hops. We now know this as the birth of the India Pale Ale. The distinction was now between ale and porter (or porter beer), as porter became the almost ubiquitous dark beverage in London during the time period. The Engineer's and Mechanic's Encyclopedia published in 1836 describes the two:
In England two distinct sorts of beer are known, called ale, and porter, or beer, and of each sort there are numerous varieties. Although the difference in the flavor of ale and of porter is sufficiently marked, it is difficult to say in what way it is produced: that it is not altogether owing to pale malt being used for brewing ale, as some assert, is clear from the fact that in many parts of the country, ale is brewed from brown malt: neither is it owing to a larger quantity of hops being used in making porter, for the pale ale which is exported in large quantities from country to India contains a larger proportion of hops than the porter exported to the same place; neither will a difference in the proportions of the malt to the water account for it, since some ales are stronger and others weaker than porter.
It appears that the distinction between ales and beers was becoming more dubious, with ale no longer a universal term for unhopped or lightly hopped beverages. In fact, by the middle of the 19th century "pale ale" and "bitter beer" were effectively synonyms. Yet, somewhat puzzlingly, the distinction of ale from beer persisted, but instead of being based on hop usage, it was now based on color. By the later parts of the 19th century, beer had become the umbrella term to describe all kinds of fermented malt beverages, and ale was simply a subdivision of beer. The Oxford English Dictionary agreed with this definition in 1884 saying under the topic of ale, "At present 'beer' is in the trade the generic name for all malt liquors, 'ale' being specifically applied to the paler coloured kinds, the malt for which has not been roasted or burnt". Further on, the colloquial usages of ale and beer gradually grew closer and closer to each other until roughly the early 1950's. If you had walked into a pub in London and asked for an ale at that time, what you would have likely received was a mild, relatively sweet, relatively low hopped malt liquor.
It was in the second half of the 20th century when ale finally started to take on the definition it has today. With the growth of lagers, both within England and around the world, lager became the beverage that most beer drinkers were familiar with. Lager would need to be distinguished from other types of beers, namely ales, and this was done on the basis of differing fermentation conditions. Lagers were defined as beers fermented at lower temperatures where the yeast tends to stay closer to the bottom of the fermentation vessel, while ales were fermented at higher temperatures with the yeast rising to the top of the fermentation vessel. By 1993, the famous beer writer Michael Jackson would declare:
In modern usage, ale indicates a brew that has a warm fermentation, traditionally with strains of yeast that rise to the top of the vessel. These 'top-fermenting' yeasts distinguish ales from lagers...
Among beer enthusiasts, beer writers and brewers there is still some debate about the technical definition of ale, but the distinction is almost always between ales and lagers as opposed to hops or color as it has been in the past. Certainly there is some overlap between ales and lager, particularly with the craft brewing scene blossoming around the world. But for the most part, the definition by Michael Jackson is accepted by people with an interest in brewing as well as people within the brewing profession.