144 Mr. Ritchie on a new photometer, &c. 
4. Place the instrument between any number of steady 
lights whose intensities are known, as for example, between 
four wax candles opposite one end, and one candle opposite 
the other, and move the photometer till the fluid remain sta- 
tionary at the division where it formerly stood, and it will be 
found that the distances are directly as the square roots of 
the number of candles; or in other words, that the intensities 
of the lights will be inversely as the squares of the distances. 
If gas lights be employed, having burners capable of con- 
suming known quantities of gas in equal times, and the pho- 
tometer be placed between them, so that the effect upon the 
air in each chamber shall be the same ; it will be found that 
the quantities of gas consumed by each, will be exactly pro- 
portional to the squares of the distances of their respective 
flames from the ends of the photometer. 
5. This instrument seems well adapted for determining 
the relative quantities of light given out by the combustion 
of coal and oil gas. Place the instrument as before between 
the two burners, and ascertain the relative intensities of the 
two lights, by squaring their distances from the adjacent 
ends of the instrument ; ascertain the quantities of gas con- 
sumed by each of the burners in the same time ; multiply 
these quantities by the squares of the respective distances, 
and the product will be the relative quantities of light, af- 
forded by the gases. Let d be the distance of the coal gas light 
and d' that of the oil gas light ; and let q be the quantity of 
coal gas consumed in a given time, and c( the quantity of oil 
gas consumed in the same time, then the intensity of the coal 
gas will be to that of the oil gas q d"^ to q' d' ^ 
6. To find the ratio between the quantities of light given 
