JF Ptak Science Books Post 2353

That is a provocative title, or chapter heading, but that's how it appears in a pamphlet I'm reading right now, a juicy thing that can lead to a memory palace of ideas...until you start reading the para below the head, when things get both more crystalline and fuzzy.

John Alexander Henderson, a lightning calculator and professor of math at Delaware College in New York, produced this sprightly pamphlet eponymously titled *Henderson's United States Intellectual and Practical Lightning Calculator, the Unity and Decimal Method,* which he published in St. Louis in 1879. (It is followed a few years later in another edition with a hundred new pages.)

What Mr. Henderson is getting at is a calculator for reducing a date to find out the day of the week a particular date falls on, from the 1st century to the 99th. On the back cover of this pamphlet he provides a tickler for this enumerating device, which is explained in another publication (Henderson's *United States Unity and Decimal Method of Calculating*).

And so in order to calculate the day of the week on which, say, November 26, 2014, occurs you would you the dial above as follows, but first an explanation of what is on the dial face:

- the outer ring contains the two-digit part of the years of a date within a century
- in the next ring are those years' "ratios"
- the next ring contains the century, from 0 to 99
- the smaller following ring are the centurys' "ratios"
- next comes the month with the ratios above in the same ring

And so, the figuring part, and bear in mind that Sunday is the first day of teh week for Mr. Henderson:

- go to the first, outer, ring and select "21" for the century, giving you a 4;
- next, the 14 of 2014 in the third ring, giving you another 4;
- then the month, November, with a ratio of 6;
- and then the simple date, 26;
- sum them up, making 40
- then divide by Henderson's holy number of 7 to find the answer of 5 and 5/7; the "exclusion of sevens" rule meaning the number of the day is the seventh's numerator, or "5". Five is fifth day of the week, making it a Thursday, which in this case, unfortunately, is incorrect (this being Wednesday). So at least this is what I can figure thus far. What good this knowledge would do me if the date was a thousand years earlier, I do not know.

There's other stuff going on in this pamphlet, including short-cut methods for business calculations and a new sort of doing arithmetic which gives me a little bit of a headache; additionally there's useful applicable bits to o how to find the square edge capacity of boards in a log, quickly figuring business interest, measuring land with a rod, and such. But most of my interest is with the calculator, for which I will have no use because it is not particularly interesting to me to know on what day of the week the Gettysburg Address was given, though it is very amusing to think about how Mr. Henderson came up with this idea.

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