Fill the bag with air by using a pair of bellows and leave it over night.  This test will show if the bag is airtight.  If it is not tight then the bag needs another rubbing.  The next operation is to fill the bag with gas.

Hydrogen gas is made from iron and sulphuric acid.  The amounts necessary for a 10-ft. balloon are 125 lb. of iron borings and 125 lb. of sulphuric acid. 1 lb. of iron, 1 lb. of sulphuric acid and 4 lb. of water will make 4 cu. ft. of gas in one hour.  Secure two empty barrels of about 52 gal. capacity and connect them, as shown in Fig. 5, with 3/4-in. pipe.  In the barrel, A, place the iron borings and fill one-half full of clear water.  Fill the other barrel, B, with water 2 in. above the level of the water in barrel A. This is to give a water pressure head against foaming when the generator is in action.  About 15 lb. of lime should be well mixed with the water in the barrel B. All

[Illustration:  Fig. 5; The Hydrogen Generator]

joints must be sealed with plaster of Paris.  Pour in one-half of the acid into the barrel, A, with the iron borings.  The barrels are kept tight while the generation is going on with the exception of the outlet, C, to the bag.  When the action is stopped in the generator barrel, A, let the solution run out and fill again as before with water and acid on the iron borings.  The outlet, C, should be always connected with the bag while the generator is in action.  The 3/4-in. pipe extending down into the cooling tank, B, should not enter into the water over 8 in.  When filled with gas the balloon is ready for a flight at the will of the operator.

** How to Clean a Clock [119]

It is very simple to clean a clock, which may sound rather absurd.  For an amateur it is not always necessary to take the clock to pieces.  With a little care and patience and using some benzine, a clean white rag, a sable brush and some oil a clock can be cleaned and put into first-class running order.  The benzine should be clean and free from oil.  You can test benzine by putting a little on the back of the hand; if it is good it will dry off, leaving the hand quite clean, but if any grease remains on the hand, it is not fit to use.

The oil should be of the very best that can be procured.  Vegetable oils should never be used.  Clock oil can be procured from your druggist or jeweler.

All loose dirt should be removed from the works by blowing with bellows, or a fan, or dusting with a dry brush; in the latter case great care should be exercised not to injure any of the parts.  Dip the brush in the benzine and clean the spindles and spindle holes, and the teeth of the escapement wheel.  After washing a part, wipe the brush on the rag and rinse in the benzine; this should be repeated frequently, until no more dirt is seen.

When the clock has dried, oil the spindle holes carefully; this may be done with a toothpick or a sliver of woodcut to a fine point.  Oil the tooth of the escapement wheel slightly, using a fine brush.