1/78,125,000 .......Jan. 15. 1/83,333,333 .......Jan. 23 1/128,834,063 ......Feb. 1. 1/226,757,400 ..... Feb. 9. 1/232,828,800 ..... Feb. 19. 1/388,200,000 ......March 7. That this diminution is not due to the air being gradually withdrawn from the walls of the gauge or from the gauge-tube, is shown by the fact that during its progress the pump was several times taken to pieces, and the portions in question exposed to the atmosphere without affecting the nature or extent of the change that was going on. I also made one experiment which proves that the gauge-correction does not increase sensibly, when the exhausted pump and gauge are allowed to stand unused for twenty days. _Rate of the pump's work_.--It is quite important to know the rate of the pump at different degrees of exhaustion, for the purpose of enabling the experimenter to produce a definite exhaustion with facility; also if its maximum rate is known and the minimum rate of leakage, it becomes possible to calculate the highest vacuum attainable with the instrument. Examples are given in the tables below; the total capacity was about 100,000 cubic mm. Time. Exhaustion. Ratio. 1/78,511 10 minutes }........ 1:1/3.53 1/276,980 10 minutes }........ 1:1/6.10 1/1,687,140 10 minutes }........ 1:1/4.15 1/7,002,000 Upon another occasion the following rates and exhaustions were obtained: Time. Exhaustion. Rate. 1/7,812,500 10 minutes }........ 1:1/3.18 1/24,875,620 10 minutes }........ 1:1/2.69 1/67,024,090 10 minutes }........ 1:1/1.22 1/81,760,810 10 minutes }........ 1:1.67 1/136,986,300 10 minutes }........ 1:1.23 1/170,648,500 The _irregular_ variations in the rates are due to the mode in which the flow of the mercury was in each case regulated. _Leakage_.--We come now to one of the most important elements in the production of high vacua. After the air is detached from the walls of the pump the leakage becomes and remains nearly constant. I give below a table of leakages, the pump being in each case in a condition suitable for the production of a very high vacuum: Duration of the Leakage per hour in experiment cubic mm., press., 760 mm. 18½ hours............................ 0.000853 27 hours............................ 0.001565 26½ hours.............................0.000791 20 hours.............................0.000842 19 hours.............................0.000951 19 hours.............................0.001857 7 days..............................0.001700 7 days..............................0.001574