The Compass in Construction

Compasses are regularly portrayed in construction images, and are also regularly ignored.

In October, 2018, the Preservation Trades Network (PTN) met at the National Park Service Historic Preservation Training Center in Frederick, Maryland, for its annual workshops.

I gave 2 presentations on the use of compasses in historic layout and design. More than 30 participants practiced their geometry - with enthusiasm. They asked good questions.

PTN workshops were held in the shops, the yard and storage bays, and an office of the Training Center. Around us were the HPTC logos.
They all included a trowel, a plane, and a compass.

As posted here: 
top: on a brochure
middle: real tools mounted on a board hung on a wall
bottom: an image posted on a bulletin board



I asked the staff who came to my presentation what they knew about using a compass. They told me: not very much.

The image of  a compass is there, along with a trowel for plaster, masonry, and stone work and plane for wood working. The compass as an equally important tool for layout and design. The understanding of how and why to use one has been lost.



 In September, 2019,  PTN joined with Historic Environment Scotland to hold the International Preservation Trades Workshops (IPTW) at the Engine Shed, Stirling Scotland. I gave 3 presentations with slides and hands-on practice drawing layouts and elevations without numbers,  using only a compass, a straight edge, and a pencil.

Our final evening we dined in the Banquet Hall of Stirling Castle, historic home of Scottish kings, including Mary, Queen of Scots, and James VI who became James I of the United Kingdom.  (No, the roof trusses were not this pink! just what my camera saw!)






 Before dinner we toured the castle which is set high above the land and the river.

It is beautifully cared for; the rooms used by the royal family furnished with fascinating furniture and tapestries depicting the time when Mary Queen of Scots and James VI lived there. The signage is excellent: clear, colorful,  with historically accurate graphics and information.

 Even jokes were in period dress: These plaques were beside the  restroom doors; a medieval beast which might be a griffin was watching from the newel post.

This poster was in the courtyard. I liked the images:  the tools in use, the appropriate clothing, and surroundings. visually interesting and easy to understand.






The text explains the work. Except for that compass the master mason holds under his arm.  It is perhaps 24 inches long, with brass fittings and an armature to set the span. This is a serious instrument.
No explanation is given for its use.





I think people simply don't understand how a compass was used. No one  asks how the project was organized, how it was planned and set down so that all the craftsmen  could reference what was to be built.

How did they share their understanding of the scope and detail? The master craftsman laid out the design of the work.
He used his compass to accurately draw and explain his plan to  the other workmen.

 I looked on the internet for the proper name of the 'armature' on a compass used in construction. 'Hinge' is sometimes used but that refers to the part where the legs join.  In the 1920's the 'arm' had become a bar with an adjusting screw; the compass called a 'spring bow".  
Because there is so little written about compasses used in construction I may need to revise this post as I learn more. 
 

Bow compasses

Page 614, Audels Carpenters and Builders Guide #2, Theo. Audel & Co., Publishers, NY, NY, 1923

Lines, in historic and modern construction

 How we build today that can be traced back to the ancient world, 
We may have forgotten how to use geometry, but we still use the concepts and tools.

Lines 
Now called Chalk Lines



Every carpenter knows :1) how to use a Line,  2) why to lay it out,  3) how to pluck the twine at the right angle.


 We learned to snap the Line from someone else, not a book.  This is hands-on teaching - master to apprentice.

We've been educating the next generation this way long before it was written down.
Theo Audel tried in 1923.
Audel's Carpenters and Builders Guide has 2 illustrations showing how to 'pluck' the Line. Other illustrations show how to set the Line with an awl.


Here is his illustration of the Line with its reel and awl.

 He explains that "The line consists of a light string or cord"  made of cotton or linen; that it can come in 20 ft., 50 ft., and 84 ft. hanks, on up to 450 - 600 ft. long lengths. That's a awfully long cord!











In 1923, tape measures were still about 5 years in the future.
We did have 6 ft long folding rules.

This one is mine: it helps me catch nuances in an existing building because I am close to what I need to document.
The tape measure is better for overall dimensions.

That word, 'Line' - the L capitalized - is often used in Practical Geometry. Serlio"s diagram and explanation is the earliest example I have found. The word was understood, not requiring any special explanation. James Gibbs says about his drawings  that they are  "Draughts of useful and convenient Buildings ...which may be executed by any Workman who knows Lines,..."

Today we use Lines for setting a wall in a straight line. The Line is along, or off-set, from where the wall needs to go. We build to it.
Rectangular foundations are 'trued' by checking with a Line that the diagonals match.


How would builder without 20th Century tools use a a Line to lay out a building?

The slave cabin of Tuckahoe is an example.

The carpenter knows how big he will make the cabin and what it will look like: 2 square boxes with roof, lofts, chimney, 2 doors and 2 windows. 

He starts with a Line (C-D) and its Perpendicular (A-B) - basic geometry that he can easily lay out with his cord.  The Lines do not yet have length, just direction.
 He chooses his length  for the width of the cabin: A-B. Probably he has a rod marked off with 5 or 10 units. See the illustration at the end of the post.
He  swings his cord  in an arc using A-B as his radius from C through B to D. Now he has the width and the length of the cabin, and 2 corners.
To find the other 2 corners, he moves to B and swings his arc from E through A to F.
He doesn't yet know where E or F are...
For that  he will stand  at D and swing the cord from A to E. Where the cord intersects with the earlier arc will be the right rear corner, E. F will be determined the same way.
Then he will check that his rectangle is true by running his diagonals F-D and C-E  If they match, he is set. If not he will adjust his Lines.
At no point does he need to use numbers. 
 
The walls of the cabin would be laid out on a framing floor with cord set from point to point, just as timber framers who work by hand do today.


Here's a 14th c. wood cut showing a rod marked in 5 units. It may be longer; it may extend behind his body to end at the rectangle with the triangle at its end - which might be for plumbing a surface.
 C. 1800 pattern books refer to 10' rods. 16.5 ' rods were used to lay out acres.





See my Bibliography for the books of Serlio and Gibbs referred to here.
See also my posts on both men, and my posts on Tuckahoe Plantation.

Audels Carpentry and Builders Guide, a practical illustrated trade assistant, Theo. Audel & Co., NYC, 1923







The subject of the next post:

 How a daisy wheel fits into the use of Lines

This daisy wheel, drawn by a compass, was on the wall of a barn in Vermont. Close examination shows that the center and the tips of the daisy's petals were regularly pricked. The radius and the diameter were regularly used as  dimensions.