Boyle described the experiment in the text accompanying his table, shown in Fig. 5.1. With considerable difficulty, he procured a glass U tube, the longer leg of which was nearly 8 ft. (2.44 m) long, while the shorter leg was some 12 in. (30.5 cm) long and was sealed at the end. He then prepared a narrow piece of paper, on which he marked 12 in. and their quarters, and he placed this in the shorter limb. A similar piece of paper, again divided into inches and quarters, was placed in the longer limb. Holding the U tube vertically, he then poured mercury into the long limb so that a column of air 12 in. long was trapped in the short limb, and the mercury levels in the two limbs were initially the same. This was the situation represented by the top row of numbers in Fig. 5.1. He then carefully added more mercury, little by little, to the long limb, and he observed the compression of the column of air in the short limb. For example, the second row of the second column of Fig. 5.1 shows that he stopped adding mercury for the second set of readings when the length of the air column was 11 1/2 in. The third column (B) shows that, at this time, the additional height of the mercury in the longer limb was 1 7/16 in. Additional mercury was then added until the air column was 11 in. high (row 3), at which time the additional height of the mercury in the long column was 2 13/16 in. Although the paper strip in the long limb was only divided into quarters of an inch, Boyle was able to interpolate and measure the height of the mercury to one-quarter of each small division (that is, 1/16 of an inch). In all, Boyle added mercury 24 times, until the length of the column of air was reduced to 3 in. (bottom row, second column A) and the additional height of the mercury was 88 7/16 in. (bottom row, column B).

Boyle added some interesting details on how he carried out this experiment. He had trouble with the breaking of the glass tubes because of the high pressures developed by the long column of mercury, so the lower part of the tube was placed in a square wooden box. This allowed him to catch the valuable mercury. As indicated above, the mercury was poured in very slowly because, as Boyle noted, it was “far easier to pour in more, than to take out any, in case too much at once had been poured in.” The long tube was so tall that the experiment was carried out in a stairwell. Boyle also used a small mirror behind the tube to help him measure the height of the mercury accurately.

As indicated above, the second column of the table (A) and third column (B) show, respectively, the length of the trapped column of air and the additional height of the mercury in the long limb (both in inches). The first column of the table (also headed A) is simply the number of quarter-inches occupied by the trapped air. In other words, it is simply the second column multiplied by 4, and it is proportional to the volume of the gas (assuming a constant cross-sectional area of the tube). In the fourth column, headed C, Boyle states “added to 22 l/8.” This is actually a misprint. The correct value is 29 1/8 in., which Boyle took to be the height of a mercury column supported by the normal atmospheric pressure. Therefore, the fifth column (headed D)is the sum of column B and 29 1/8 in., except that all the fractions in column D are given in 16ths. This column shows the pressure to which the bubble of gas was subjected. The last column (E) is the calculated pressure for the volume shown in the first column (A) according to Boyle’s hypothesis that volume and pressure are inversely related. The fractions are difficult to read and are listed in the Fig. 5.1 legend.

The first thing that strikes today’s reader is the extremely awkward fractions such as 2/17, 10/13, and 18/23. How did Boyle end up with such strange numbers? The answer is that he used simple multiplication and division and kept the vulgar fractions. As an example of how the pressures in column E were calculated, consider row 6, where the value in the first column (A) is 38. The hypothesis is that P₁V₁ = P₂V₂ or P₂ = P₁V₁/V₂, where P is pressure and V is volume. The first row shows that P₁ is 29 2/16 while V₁ is 48. Because V₂ is 38, the expression is (29 2/16 × 48)/38 which gives P₂ = 36 15/19 in. of mercury (see Appendix).