On March 28, 1885, Isaac wrote in his journal: "[Doc Dix]...brought coffee boxes down for making incubator." It was the first of several entries in his journal about using the wooden boxes in which coffee was delivered for Dix's little store at their home, where the local post office was also located. Finally, on May 9th, Isaac recorded: "I up to Dix, put plaster into incubator." From the entries, I assume he used the wooden box for the basic container, creating some sort of racks or trays to hold the eggs, and plastering the interior to allow use of a candle or some other source of heat to warm the eggs. Perhaps the plastered box was placed on a shelf above the stove, but that seems less likely if Mrs. Dix were incubating eggs in late spring. Unfortunately, Isaac provided no such details.
Eggs should be incubated at a temperature between 99 and 102 degrees Fahrenheit, with 99.5 considered the ideal. It is unlikely that Isaac had the means to provide such specific temperature for his incubator. In addition, eggs should not be kept in a tightly sealed container, for proper aeration and gas exchange between the embryo and the environment outside the shell are important.
Chicks will generally hatch after 21 days of incubation. During that time it is desirable to turn the eggs at least twice a day, more frequently if possible. This will exercise the embryo and help avoid having the embryo stick to the shell. It is preferable to incubate eggs with the pointed end down, so turning and tipping should return the eggs to that position unless they are turned more often. When it gets close to hatching time, laying the eggs on their sides is preferable. Of course, many modern incubators have mechanical equipment to accomplish the necessary turning, but surely Isaac would not have included such conveniences in his invention.
After 7 to 10 days the eggs can be examined to determine whether embryos are developing inside the shells. The process involves using a bright light behind the egg in a dark room to get a shadow image of the developing embryo through the shell--or the absence of any such shape if no embryo has developed. This is called candling, since a candle was the first method of providing the necessary light. It is not unusual for about half of the eggs to fail to produce an embryo, depending on whether the rooster in the flock provided fertilization for the egg, as well as whether the incubating process has succeeded.
The easiest method for hatching chicks is to have a "broody hen" and let her do all the work of keeping the eggs warm and turned! Eventually, that is the method Isaac relied upon, but his original flock was acquired by hatching chicks in his self-designed incubators made for himself and Mrs. Dix from wooden coffee boxes.