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Rob,

Thank you for the reference. Dierick Ruyter’s booklet is clearly the source of the illustration I attached to my post. However as I noted, the scales shown for the backside do not appear to be consistent with Aspley's description of them. I admit however that I have not really made much of a great effort to muddle through the old English and produce an accurate reconstruction of them. I invite others to give it a try.

I would also note that van Poelje https://eeuwen.home.xs4all.nl/images/Pics/Kombuispraat/Pleinschaal/200404%20Gunter%20Rules%20in%20Navigation%20V13-1.pdf states "Plains-Scales are called Gunter rules, when at least the NUM scale is included in the set of scales." The NUM scales are 2 cycle logarithmic scales and hence, according to this definition, Ruyter shows a Gunter rule rather than a simple Plain Scale.

Thank you for the offer of a scan of Ernst Crone's article but unless an OCR version is available that could be subjected to machine translation I doubt that I could get much out of the Dutch version. Please don't go to any trouble.

This is dangerously off topic but I was struck by the changes in spelling (particularly by the use of u and v), capitalization and wording between Aspley's 1624 and 1647 editions

1624

I haue likewiẜe cauẜed two other lines to be placed vpon the backeẜide of the Scale , which I doe call the firẜt and ẜecond lines of Longitudes : the firẜt is diuided into 20. vnequall parts, or leagues , which 20 leagues are equall vnto the coard of 90.  The vẜe of this firẜt line of Longitudes , is to fhew how many leagues and miles in any Parallell, doe anẜwere vnto one degree of the Equinoctiall.

The ẜecond line of Longitude is diuided into 100 proportionall parts , or into 100. vnequall leagues; and euery league is fubdiuided into miles, and halfe miles. The vẜe of this line is thus: when you haue found by the firẜt line of Longitudes, how many leagues and miles doe anẜwere vnto a degree of the Equinoctial in any latitude you deẜire: this ẜecond line will ẜhew you how many degrees any number of leagues in that parallell , will anẜwere vnto, in Equinoctiall circle.

1647

I have likewiẜe cauẜed two other lines to be placed upon the back ẜide of the Scale , which I doe call the firẜt and ẜecond Lines of Longitudes : the firẜt is divided into 20 unequall parts, or leagues , which 20 Leagues are equall unto the Cord of 90.  The uẜe of this firẜt Line of Longitude , is to fhew how many Leagues and Miles in any Parallell, doe anẜwere unto one degree of the Equinoctiall.

The ẜecond Line of Longitude is divided into 100 proportionable parts, or into 100 unequall Leagues; and every league is ẜubdivided into miles , and halfe miles. The uẜe of this Line is thus when you have found by the firẜt Line of Longitudes , how many Leagues and Miles doe anẜwere unto a degree of the Equinoctial in any latitude you deẜire: this ẜecond line number of will ẜhew you how many degrees any leagues in that Parallell , will anẜwere unto a degree in the Equinoctiall Circle.

Regards,
Robin