54 OPTICAL PROJECTION 



portance that the reader should understand this, and the 

 terms in which pressure is described. 



Pressure is stated as so many inches ' ; meaning of 

 water. This is measured very simply by what is called a 

 U-tube, shown in fig. 32. A glass tube is bent into U form, 

 deep enough to measure any pressure likely to 

 be met with say eighteen inches from the 

 top to the bend. One end is corked, with a 

 bit of tube passing through the cork on which 

 can be stretched a vulcanised tube from the 

 bag ; and a scale of inches is drawn between 

 the legs of the U, measured from a zero-line 

 across the centre. Water is poured in at the 

 open end till it stands level at the zero-line. 

 Then the tap from the bag is opened, and the 

 pressure sends down the water in that arm of 

 the U, driving it up the other by an equal 

 amount ; and the difference of levels is the 

 pressure of the gas. House-gas in London is generally rather 

 under two inches, i.e. it drives the water down 1 inch on one 

 side, and up the other. A gas-bag of 36 x 24 x 24 size, under 

 1 cwt. pressure, when drum-tight, may have almost any pres- 

 sure, as the tension of the bag is added ; but when this is 

 gone off after a few minutes, the pressure is usually about 

 9 inches ; when the bag is about three-quarters empty, this 

 is generally gone down to about 4^ or 5 inches. 



It will show the kind of danger to be guarded against, to 

 describe the only explosion a very slight one which ever 

 happened to myself, almost at the beginning of many years' 

 use of the mixed jet. I was using up half a bag of oxygen 

 in determining a ticklish experiment, and had filled the 

 coal-gas bag considerably too full in proportion, so that it 

 was quite one third full when the oxygen was nearly ex- 

 hausted. Absorbed in the experiment, I did not notice that 

 the board was practically ' down ' upon the flattened oxygen 



