From: kitchel@moray.dpsi.com (Sidney W. Kitchel)
Subject: Re: YABB - DIFFERENCE ENGINE - Spoiler
Organization: Data Parallel Systems, Inc
Date: 24 Jul 92 14:36:38 GMT

ccaajmf@ucl.ac.uk (John Franks) writes:

>In article <kitchel.711901699@moray> kitchel@moray.dpsi.com (Sidney W. Kitchel) writes:
>>jaguar@wiz.outb.wimsey.bc.ca (Jeremy Reimer) writes:
>>
>>	Let's be a little clearer. Are you claiming that Charles
>>Babbage could see into the future or that he was a time traveler??
>>How could he plan a "forerunner"???? The Difference Engine was not a
>>mere calculator. It was specifically a mechanical device capable of
>                                                          *******
>>being commanded to execute calculus of differences computations and
>       *********
>Be careful. You're implying the difference engine was programmable in some
>way; wrongo. It just did the difference  computations. Data in at one end,
>answer out at the other after you turn the handle.

	It is interesting that in my original sentence I wrote
"programmed" instead of "commanded", but changed it because I thought
it too strong. A more accurate way of putting it would be to say that
the Difference Engine could be set up for certain initial conditions
and certain differences at certain orders of difference.

>>>JR> Babbage had plans for the Analytical Engine to have a "CPU", memory,
>>>JR> storage,
>>>JR> etc, and this more like the stuff in the novel.
>>
>>	It would be, if that is what Babbage did. But once again, he did
>>not know what electo-mechanical and electronic computers were going
>>to become in the 20th century. The Analytical Engine had registers, an
>>arithmetic "mill" and was commanded via Jacquard Loom-style cards.
>>There was no real memory or stored programs. There was no "CPU".
>>It was 100% mechanical. It was slow and prone to lock up because of 
>                         ***********
>>the great number of gears.
>>
>Careful again. It wasn't built; only the arithmetic unit was built, and about
>the most impressive thing it could do was compute multiples of pi. 
>  As for speed, well, it was faster than any number of humans, I suspect. 
>  And there's no stigma in being mechanical.
>  The stored program issue is a dodgy one; as I remember, it had two inputs,
>so it could handle program & data seperately. I also believe it had conditional
>execution; basically, I think you'd probably find it was equivalent to a 
>Turing machine, or lambda calculus or whatever, and therefore qualifies as a
>computer. I stand to be corrected. My lecturer would never quite be drawn on
>the conditional execution bit... 

	I was not trying to claim that the Analytical Engine is not a
computer. Rather I was cautioning against reflecting our ideas of
computers back onto a precursor, which should be examined on its own
merits and in its own context.

>  On the gear problems I give way, but the difference engine worked pretty
>smoothly when they built it at the Science Museum (London, recently).

	Unfortunately, Babbage was not as lucky. He was quite
disappointed to discover that his Difference Engine would sometimes
bind a gear at a point in the computation, but not stop. This would
introduce an error in the result. The error would be printed and be
just as hard to find as the errors in manually calculated tables.

							--Sid

--
Sidney W. Kitchel  	    kitchel@cs.indiana.edu, kitchel@dpsi.com
Data Parallel Systems, Inc.  ============||  DPSI  ||===============
4617 E. Morningside Drive	                      (812) 334-8100
Bloomington, Indiana,  47408  USA		FAX:  (812) 334-8121