400 



Mr. C. H. Giraingham on a new 



[Dec. 7 



perfect control, c is a simple stopper to prevent the sulphuric acid in 

 a from absorbing moisture. 



We now come to the jet (g, fig. 1) : it is made with a double-stopper 

 arrangement, so that the jet may be easily separated from the pump, 

 should it require altering, or any stoppage occur. 



The jet itself, where the mercury divides, is made of platinum and 

 blown to the glass ; the centre hole is straight, the two side ones being 

 drilled at a slight angle, to direct the mercur}^ into the side tubes. 



Owing to the double stopper the jet may be turned, at any time, so as 

 to direct all three streams of mercury down the centre tube if desired. 

 Whether any advantage is gained by so doing is perhaps rather doubt- 

 ful, though I think that a great force of mercury will sometimes carry 

 entirely down minute traces of air, which a less force will only take part 

 of the way down, the little bubble then rising again to the surface, when the 

 work has to be recommenced. 



At first the division of the one column of mercury into three streams 

 presented rather a difficulty ; but after numerous experiments I found a 

 platinum jet most simple and successful. 



The spiral (W, fig. 1) is used as a flexible joint between the apparatus 

 and the pump ; it enables one to use levelling-screws, &c. without fear of 

 breaking the connecting-tube. Of course I am speaking with the under- 

 standing that all instruments are put on the pump by means of a mer- 

 cury or blown joint, as no other is perfect when working at the highest 

 exhaustions. India-rubber connexions, covered with glycerine, are very 

 excellent joints, but are very disagreeable to w^ork with, and cannot be 

 compared with a blown or mercury joint. 



The gauge fig. 1) is fixed to the pump by a mercury joint in order 

 that it may be easily removed and cleaned when necessary, accurate 

 heights being much more easily taken with a perfectly clean surface of 

 mercury. 



The height of the gauge and barometer is taken by means of the slide 

 y moving rather stiffly on the divided rod ; the slide carries an arm pro- 

 jecting across both the gauge and barometer at perfect right angles to 

 the divided rod. 



Behind the top part of these instruments is placed a mirror, in order 

 to prevent the interference of parallax while taking the heights, by using 

 the reflected images to level the eye. A cathetometer is of course pre- 

 ferable ; but very great accuracy may be obtained in this way. 



Just above the tap at the end of the sulphuric acid reservoir is placed 

 an electrical vacuum-tube (D), the terminals of which are made of alu- 

 minium, and only separated by one eighth of an inch. It is very con- 

 venient to have this fixed to the pump, as it is thus always in readiness 

 to test the vacuum as regards its conductivity with the induction-coil. 



Below this tube, and between it and the tap, is placed a small bulb (E) 

 packed rather tightly with gold leaf, intended to stop any mercury v apour 



