The 5th column of the above table, or weight of aqueous vapour, is new, and may therefore require explanation. Gay Lussac is considered the best authority in regard to the specific gravity of steam; but it would be well if his results were confirmed or corrected, as they are of importance. According to his experience, the specific gravities of common air and of pure aqueous vapour, _of the same temperature and pressure_, are as 8 to 5, or as 1 to .625. Now I assume that 100 cubic inches of common air, free from moisture, of the temperature 60° and the pressure of 30 inches of mercury, weigh 31 grains nearly. It is an extraordinary fact that philosophers are not agreed upon the absolute weight of a given volume of common air. Most authors now assume the weight of 100 inches = 30.5 grains, whilst according to my experience it is more than 31 grains. If common air be assumed 31 grains, steam would be 19⅜ grains for 100 cubic inches, at the same temperature and pressure, could it subsist; but as it cannot sustain that pressure at the temperature of 60° we must deduct according to the diminished pressure, the utmost force of steam at 60° being .65 parts of an inch of mercury, we have 30 inches ∶ 19⅜ grains ∷ .65 ∶ .420 grains = the weight of 100 cubic inches of aqueous vapour at 60° and pressure .65 parts of an inch; which is the number given above in the table. The like calculation is required for any other pressure: but in addition to this, there is to be an allowance for the temperature from the 2d column: Thus, let the weight of 100 cubic inches of steam at 32° be required. We have 30 inch. ∶ 19⅜ grs. ∷ .26 inch. ∶ .1679 grs.; the weight of 100 inches of steam at 60°; then if 480 ∶ 508 ∷ .1679 ∶ .178 grs. = weight of 100 cubic inches of steam at 32° and pressure .26 parts of an inch, the tabular number required. _Examples._ 1. How many cubic inches of air at 60° are equivalent in weight to 100 cubic inches at 45°? By the column headed _volume of air_ we have this proportion, if 493 ∶ 508 ∷ 100 inch. ∶ 103.04 inches, the volume required. 2. How many cubic inches of air with the barometer at 30 inches height, are equal in weight to 100 cubic inches when the barometer stands at 28.9 inches? _Rule._ The volume of air being inversely as the pressure, we have, 30 ∶ 28.9 ∷ 100 inches ∶ 96⅓ inches the answer. 3. How many cubic inches of dry air are there in 100 inches saturated with aqueous vapour, at the temperature of 50°, and pressure 30 inches of mercury? Here the formula (_p_ - _f_) ----------- _p_ applies, where _p_ denotes the atmospheric pressure at the time, and _f_ denotes the utmost force of vapour in contact with water at the temperature. Hence _p_ = 30, _f_ = .49 per table, and we have (_p_ - _f_) (30 - .49) 29.51 ----------- = ----------- = ----- = 98¹¹/₃₀, or _p_ 30 30 98¹¹/₃₀ percent dry air. & 1¹⁹/₃₀ vapour. --------- 100.