Table 11

1

50.00

2

41.42

3

36.61

4

33.33

5

30.90

6

28.98

7

27.43

8

26.12

9

25.00

10

24.04

11

23.17

12

22.40

13

21.71

14

21.09

15

20.52

16

20.00

17

19.52

18

19.07

19

18.66

20

18.27

21

17.91

22

17.57

23

17.25

24

16.95

25

16.67

26

16.40

27

16.14

28

15.90

29

15.66

30

15.43

31

15.22

32

15.02

33

14.83

34

14.64

35

14.45

36

14.28

37

14.12

38

13.96

39

13.80

40

13.65

The distance x (equal to a c in Fig. 95) for any candle-power (c. p.) for any distance L between centers of the lights, may be computed by dividing L by 1 plus the square root of the candlepower; thus, x = 1 +√c.p.

261. In using the photometer, care must be taken to prevent the entrance of light into the sight-box from any other source than the lights which are to be compared. A screen of black velvet should be suspended behind each light to prevent any light from being reflected towards the sight-box.

It is not necessary to have a darkened room to operate in if the instrument is properly protected with curtains and screens of black cloth.

In testing gas, the pressure must be kept uniform, and the rate of combustion should be carefully measured. Standard candles can be obtained from the American Meter Co., New York.

The candlepower of ordinary illuminating gas is measured while burning at the rate of 5 cubic feet per hour, under a pressure of .5 inch of water. To secure very exact measurements, small corrections must be made for the temperature of the gas and for the moisture contained in it.

The candle should always be weighed before and after each test, and allowances must be made in computing the candle-power of the light under examination, if the rate of consumption of the candle varies either way from the standard rate of 120 grains per hour.

Heating and Ventilation of Buildings.