The Screw-Neck Jar: A Thread That Stops Short of the Seal

"The thread at both ends is made to vanish into the neck before it reaches the top or the bottom thereof." "The advantage of the first is that, if the thread of the screw extended to the upper end, it would be marred by the grinding to finish the end of the neck, and would present sharp points which would cut and injure the cap intended for it". "And the advantage of the second is that, if the screw-thread, as heretofore, should extend to the shoulder of the bottle, the screw-cap would be interrupted in coming down to the shoulder, and the india-rubber washer could not fit closely to its position, and thus the air-tight qualities of the bottle or jar would be impaired."

Look at a Mason jar's neck. Measure the smooth band between the top of the thread and the rim, and the gap between the bottom of the thread and the shoulder. Count how many turns the cap takes to seat. Warm the jar with hot tap water so the glass does not crack, then fill it with water just off the boil to 2 cm below the rim. Put on the lid and screw the band down finger-tight. Leave it to cool for an hour and read the water's temperature through the glass with the thermometer against its side. Press the middle of the lid: a sealed lid is pulled down and does not flex. Unscrew the band and lift the lid by its edge to feel how hard the air holds it.

**John L. Mason** of New York received US 22,186, dated 30 November 1858. He wrote that he had already patented screw-tops and caps, and that moulding the jar in glass let him make improvements "which could not be produced in spinning". **Honest limits.** The notebook assumes the headspace is only air and water vapour and ignores the lid's own springiness. The air share and lid sizes are examples. Your pressed lid is the measurement.