
A younger family member saw the case containing the slide rule I have had since my school days. He asked what it was. His eyes widened as he took the slide rule out of its case. What is it? He asked.
“It’s a slide rule”, I replied. What is a slide rule? He asked.
I explained that it was an analogue computer accurate to three significant figures.
“Show me how it works”, he asked. So, I showed him some simple calculations using the slide rule. He wanted to know more than I remembered. “I will have to read the instructions”, I said. It was then I remembered I had lost them many years ago. “No problem”, I said. “I will get them off the Internet.” I actually got them from Amazon.1
Konshak’s book contains a facsimile of the instructions for my slide rule (Thornton model 271 pictured above). They were written in 1965. While reading the instructions and working through the examples, I couldn’t help noticing the em dash’s use and abuse, and other solecisms.
There are three types of dashes in British English:2
- The hyphen. Hyphens make links. They form compound adjectives such as a three-year-old child, computer-based work, or short-term goals.
- The en dash is longer than a hyphen, but shorter than an em dash. An unspaced en dash is often used to replace to in a range of dates or figures, for example, 1964–1971 or 75–86, but is optional. However, there is no dedicated en dash key on a UK computer keyboard, so one has to rely on shortcut.
- The em dash is longer than the en dash and joins two words. For example, word—word. When used in pairs, em dashes can draw special attention to the phrase they surround. Its use is optional, although Oxford University Press favours using the em dash. UK typewriters and computer keyboards have no dedicated em dash key, which is why, except for Oxford University Press, British English rarely uses it.
In American English, the hyphen and en dash serve the same functions as they do in British English.
In American English, the em dash has four uses:3
- To summarise words, phrases, or clauses that summarise a previous list.
- To set off a series as an appositive.
- For a sentence interruption that adds to the meaning of the sentence but is not grammatically part of the sentence.
- To show an unfinished statement.
However, its use is optional. Hemingway, for example, did not use em dashes, at least not in The Snows of Kilimanjaro.
I am not a fan of the em dash, although it has its place if used properly. Often, a pair of commas or a pair of brackets (parentheses) would do just as well. Cutts4 quotes from the Times style guide:
“Dashes are sloppy punctuation, ugly in narrow columns of newspaper type. They often indicate that a sentence is badly constructed and needs rewriting.”
I agree with that statement and extend it to all writing. However, there is another reason why I am not a fan of the em dash. That is, their frequent use in AI-generated content can result in human-created content sprinkled with em dashes being treated as AI slop.
Strunk and White (who appear not to recognise the en dash) advise using the (em) dash “only when a more common mark of punctuation seems inadequate”.
My bête noire is the em dash that does not join two words, for example, word — word.
Back to the slide rule instructions, which, as I say, were written in 1965. The instructions contain three pairs of em dashes that could, and in my view, should, have been replaced by pairs of commas. The most egregious solecism, however, was an em dash that did not join two words. A simple comma would have sufficed.
Strangely, there was no full stop at the end of any sentence save for one that ended ‘etc.’, where a full stop at the end of the sentence was not needed.
As an instruction manual, it achieved its goal of teaching me how to use the slide rule. Reading the instructions sixty-one years after they were written, I found their sloppy punctuation and badly constructed sentences had not aged well.
Those instructions are now a footnote in history. The invention of the first handheld electronic scientific calculator5 in 1972 sounded the death knell of the slide rule business worldwide, and by 1980 British Thornton, along with every other slide rule manufacturer, had ceased making slide rules.
When my young relative next visited me. I showed him how to use the slide rule. By then, though, his interest had waned. My explaining that the slide rule was invented in 1630 and used by scientists and engineers for nearly 450 years to design buildings, bridges, ships, and aircraft evoked little interest. That is, until I told him that NASA used slide rules to calculate the flight path of the Apollo spacecraft, as depicted in the 1995 movie Apollo 13,6 and that every Apollo spaceflight carried one to do fuel calculations as a backup to the onboard computer.7




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