been attended with the slightest accident, and owing to the presence of the four leveling-screws, the pump when righted returns, as shown by the telescope of the cathetometer, almost exactly to its original place. In the inclined position the exhaustion of the vacuum bulb is accomplished along with that of the rest of the pump. The exhaustion of the vacuum-bulb when once effected can be preserved to a great extent for use in future work, merely by allowing mercury from the reservoir to flow in a rapid stream at the time that air is allowed to re-enter the pump. During the first process of exhaustion the tube of the gauge is kept hot by moving to and fro a Bunsen burner, and is in this way freed from those portions of air and moisture that are not too firmly attached. After a time the vacuum-bulb ceases to deliver bubbles of air; it and the attached tube are now to be heated with a moving Bunsen burner, when it will be found to furnish for 15 or 20 minutes a large quantity of bubbles mainly of vapor of water. After then production ceases the pump is righted and the exhaustion carried farther. In spite of a couple of careful experiments with the cathetometer I have not succeeded in measuring the vacuum in the vacuum bulb, but judge from indications, that is about as high as that obtained in an ordinary Geissler pump. Meanwhile the various parts of the pump can be heated with a moving Bunsen burner to detach air and moisture, the cement being protected by wet lamp-wicking. In one experiment I measured the amount of air that was detached from the walls of the pump by heating them for ten minutes somewhat above l00° C., and found that it was 1/1,000,000 of the air originally present. I have also noticed that a still larger amount of air is detached by electric discharges. This coincides with an observation of E. Bessel-Hagen in his interesting article on a new form of Töpler's mercury-pump (Annalen der Physik und Chemie, 1881, vol. xii.). Even when potash is used a small amount of moisture always collects in the bends of the fall tube; this is readily removed by a Bunsen burner; the tension of the vapor being greatly increased, it passes far down the fall-tube in large bubbles and is condensed. Without this precaution I have found it impossible to obtain a vacuum higher than 1/25,000,000; in point of fact the bends should always be heated when a high exhaustion is undertaken even if the pump has been standing well exhausted for a week; the heat should of course never be applied at a late stage of the exhaustion. Conversely, I have often by the aid of heat completely and quickly removed quite large quantities of the vapor of water that had been purposely introduced. The exhaustion of the vacuum-bulb is of course somewhat injured by the act of using the pump and also by standing for several days, so that it has been usual with me before undertaking a high exhaustion to incline the pump and re-exhaust