The luminous intensity given by a petroleum lamp varies considerably with the quality of the oil employed, and also with the quality, the state of cleanliness, and the cut of the wick. A badly-trimmed wick, or one not cut level, gives, for example, a flame which on one side is too long, and slightly yellow or brown, because the supply of air at that point is insufficient, and on the other side is perhaps too small, the air supply being there in excess. Now, it is well known that under both of these conditions the illuminating power suffers.

The management and regulation ot wicks is delicate work, and certainly ought to be included in an estimate of the expense of the lamp.

Another source of expense in using this class of illuminant is that arising from the use of the lamp - repairs, consumption of wicks and chimneys, losses of oil, and waste used for cleaning. (Table I.)

It may be estimated that these sources of expense amount per lamp and hour to-

Expense of lamp .

•0950d

Wicks, chimney, etc.

•0427d.

•1377d

Finally, to include the additional trouble connected with the use of this illuminant, such as purchasing the oil, storing oil, risk of fire, etc, we think that the theoretical price ought to be increased by 25 per cent. It will be observed that the luminous intensity diminishes considerably when the photometric measurement is made at an angle of 45°.

Table I. - Petroleum

Chief Types of Lamp.

Actual

Expense,

25 per cent.

in excess of calculated.

Angle with Horizon. 0°. '

Angle 45°.

Candle power..

40

29.3

• *

Consumption of petroleum of best quality 800 grammes per litre ..

90 grm.

90

• •

Consumption per candle power per hour..

2.25

3.037

3.796

Expense per candle per hour, at 1 1/2. per litre

• •

•005396d.

•006745d.

Working expenses per candle per hour..

• •

•00465d.

•00465d.

Total expenses per candle per hour .

• •

•010051d.

•01140d.

Total expenses per lamp per hour, 29 candle power at 45

• •

• 2973d.

•33677d.

According to the experiments of Heim, the loss of illuminating power is proportional to the diameter of the burner. It is 20 per cent, for burners with an ordinary long flame; about 35 per cent, for the 30 mm. burners used in lamps which give an intense round flame,' and amounts to 5 per cent, with 60 mm. burners. It will be possible to compare the luminous in-tensities of the various systems of lamps and burners at this angle of 45°. The one class giving the maximum intensity at 0° (horizontal rays), the other at 90° (vertical rays).

Moreover, lamps are usually employed under such conditions that their rays are at this angle.