It was a pretty easy thing finding Sarah L. Cummings in the 1900 and 1880 census. Many of the letters she had kept still had their addressed envelopes, even though a stamp collector at some time in the past had surgically removed the stamps. The image below is a tiny section of the 1900 Federal Census for Rumney, Grafton, New Hampshire.
I know it's a little hard to read, but it lists William H. Cummings, born February 1841 in New Hampshire; and, his wife Sarah L. Cummings, born July 1841 in Massachusetts. The census taker was also instructed to ask the informant how many children the wife had given birth to, and of those, how many are still living. In this case the answer was Sarah had had two children and both were still living in 1900.
Unfortunately for us, the 1890 Federal Census was lost to fire in 1921. So the next available census is 1880. Again, the envelopes of the correspondence Sarah received help identify her location. The bulk of the correspondence was addressed to Easton, Pennsylvania and it is exactly where we find her. The image below is a fragment of the 1880 Federal Census for Easton, Northampton, Pennsylvania.
This image is a little easier to read. William, who is listed as a rail road conductor, is living with his wife and their two children. Whenever you are using the census records for your research, always keep in the back of your mind that the census takers are human. They can make mistakes just like you and I. This is a great example. The oldest child, Winfred H., who is nine years old is listed as a son. He is actually a she. Both of their children are listed as being born in Pennsylvania so William and Sarah must have moved to Pennsylvania some time before 1871.
The 1870 Federal Census for South Easton, Northampton, Pennsylvania does list the couple. They have no children and William is listed as a Conductor. I would have attached an image for this record, but the image quality is so poor you would never have been able to see it.
Another question the 1900 census taker asked was how many years a couple had been married. In William and Sarah's case the answer was 39 years. This places their marriage in 1861. This would mean Sarah married when she was twenty years old and in all likelihood this was her first marriage.
So now I need to figure out what her maiden name is...