I have some other county historical statistics on my Slatington News project site.
Population Growth of Slatington, Pennsylvania from 1870 to 2010. Since about 1910, population in the borough has been relatively stable. This is partly due to the fact that once the town was built up within the legal boundaries of the borough, there was not that much growth that occurred. (I should add that there were some additions to the borough's boundaries over the years which did expand acreage within the town.) While the number of houses within the borough has remained pretty consistent, there has been a sizable decrease in available hotel (almost nonexistent now) and apartment housing that has been lost. From looking at the figures, you can also see evidence of the postwar baby boom, as the town's population peaked in 1970, and since then it has declined. Part of that decline is also probably due to an aging of the borough's population, although I have no firm figures on that. (The chart is also available as a pdf.)
Percentage of Slatington’s Population Born in PA from 1870 to 1940. The data shows that in this period of time the town remained populated by largely inhabitants born in Pennsylvania, and the percentage increased from about 77% to about 93% reflecting an increasingly homogeneous population (based on place of origin). There was only a small influx of immigrants from the British Isles (many from Wales) into the town, and that was mostly in the 1880s and 1890s to work in the slate industry. There was no real flood of immigrants into the town from 1900 to 1920 when immigrants were pouring into the US. (This chart is also available as a pdf.)
Percentage of Slatington's Population Born in Wales from 1870 to 1940. Immigrants from Wales were a major labor force in the slate industry as it developed around the town in the late nineteenth century, and the data show the particularly large percentage of inhabitants of the town who had come from Wales by 1900. Over the ensuing decades as these immigrants died, Slatington became more and more a town of inhabitants born in Pennsylvania. (This chart is also available as a pdf.)
Changing Age Structure of Slatington from 1870 to 1940. In 1870, almost sixty percent of the borough's population was over the age of eighteen. That number decreased as people poured into town, found work in the slate industry, and had kids. That slowly reduced the average age in the town, but by 1910, the town's population had again begun to age. By 1940, the figure was much different, with almost seventy-two percent of the town's residents being over the age of eighteen. It was a "mature" town to say the least. (This chart is also available as a pdf.)
In conclusion, for the seventy-year period, Slatington expanded in size, initially quite rapidly, as its slate industry flourished, but then the population stabilized and aged. The town also remained a rather closed society based on place of origin.