ADDENDA.
1. French barometers are graduated to millimetres. An English inch is equal to 25·39954 millimetres. Hence, 30 inches on the English barometer scales correspond to 762 millimetres on the French barometer scales. Conversions from one scale to another can be effected by the following formulæ:—
| (1) Inches = millimetres divided by | 25·39954 | |
| (2) Millimetres = inches multiplied by | 25·39954 |
Of course, a table of equivalent values should be drawn up and employed, when a large number of observations are to be converted from one scale to the other.
2. In Germany, barometers are sometimes graduated with old French inches and lines,—the vernier generally indicating the tenth of a line.
Old French Lineal Measure.
| English Inches. | |||||
| 1 | douzième, or point | 0·0074 | |||
| 12 | points | = | 1 ligne | = | 0·0888 |
| 12 | lignes | = | 1 pouce | = | 1·065765 |
| 12 | pouces | = | 1 pied | = | 12·7892 |
| 1 | pied | = | 324·7 millimetres. | ||
“The Germans indicate inches by putting two accents after the number; lines, by putting three accents; 27″ 3′″·85, means 27 inches 3 lines 85 hundredths of a line; more frequently, they give the height in lines, and the preceding number becomes 327′″·85.”—Kaemtz.
3. Rule for finding Diameter of Bore of a Barometer Tube.
“If the maker has not taken care to measure the interior diameter directly, it may be deduced from the exterior diameter. The exterior diameter is first measured by calipers, and, by deducting from this diameter 0·1 of an inch for tubes from ·3 to ·5 of an inch in external diameter, we have an approximation to the interior diameter of the tube.”—Kaemtz.