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The brass scale is logarithmic and each line is marked with a speed starting at 5mph with a maximum of 55mph underneath the top line.
I don`t think the speed scale is logarithmic. Simple physics suggests that the pressure and hence the column height should be proportional to the square of the speed. You can`t see where the zero is, but for example the difference between the 20 and 40 marks (40 x40 minus 20×20 = 1200) should be half that between the 10 and 50 marks(50×50 minus 10×10 =2400). Scaling off a computer screen isn`t easy, but it looks about right.
I have just realised that I picked a daft example to illustrate that height is proportional to speed squared, since 40 minus 20 is also half of 50 minus 10. ie a linear relationship would also fit. I`ll try again:
Measuring height differences off my computer screen I get 10mph to 30 mph 6.8 cms
10 to 40 12.9cms
10 to 50 20.7 .cms
With a speed squared law the heights should be in the ratio 800 to 1500 to 2400 or 1 to 1.875 to 3 (With a linear law it would be 1 to 1.5 to 2
The actual ratio I measure is 6.8 to 12.9 to 20.4 or 1 to 1.9 to 3.05
Looks near enough
Sorry for the mistake
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