(1) Openings directly opposite each other cause a rapid circulation of air or a "direct draught".
(2) Indirect openings cause a slower circulation of air or an "indirect draught".
2. By a cross current of air which tends to check the draught.
FUELS
A discussion of the fuels may next be taken. With pupils of Form IV it will not be wise to go into too many details regarding these. Besides the classification of the commonest ones, they may be compared from the standpoints of cost, and of the time and labour required in their use.
Classes of Fuels:
Liquid—coal-oil, gasolene, alcohol
Solid—coal (coke), wood (charcoal)
Gaseous—natural gas, coal gas.
Note.—Electricity is a means of producing heat, but cannot be called a fuel.