When all the fittings, burners excepted, have been connected, the whole system of pipes must be tested by putting it under a gas (or air) pressure of 9 or 12 inches of water, and observing on an attached pressure gauge whether any fall in pressure occurs within fifteen minutes after the main inlet tap has been shut. The pressure required for this purpose can be obtained by temporarily weighting the holder, or by the employment of a pump. If the gauge shows a fall of pressure of one quarter of an inch or more in these circumstances, the pipes must be examined until the leak is located. In the presence of a meter, the installation can conveniently be tested for soundness by throwing into it, through the meter, a pressure of 12 inches or so of water from the weighted holder, then leaving the inlet cock open, and observing whether the index hand on the lowest dial remains perfectly stationary for a quarter of an hour--movement of the linger again indicating a leak. The search for leaks must never be made with a light; if the pipes are full of air this is useless, if full of gas, criminal in its stupidity. While the whole installation is still under a pressure of 12 inches thrown from the loaded holder, whether it contains air or gas, first all the likely spots (joints, &c.), then the entire length of pipe is carefully brushed over with strong soapy water, which will produce a conspicuous "soap- bubble" wherever the smallest flaw occurs. The tightness of a system of pipes put under pressure from a loaded holder cannot be ascertained safely by observing the height of the bell, and noting if it falls on standing. Even if there is no issue of gas from the holder, the position of the bell will alter with every variation in temperature of the stored gas or surrounding air, and with every movement of the barometer, rising as the temperature rises and as the barometer falls, and _vice versâ_, while, unless the water in the seal is saturated with whatever gas the holder contains, the bell will steadily drop a little an part of its contents are lost by dissolution in the liquid.