300 



Analysis of Scientific Books, 



In the annexed figure, ABCD and EFGH 



are the cylinders, AB and FG the plates 



of glass. CD, EFG the ends shut up by the 



circular tin plates, the blackened paper is 



represented by the lines between AB, CD 



and EH, FG. The other parts will be ob- 

 vious from the mere inspection of the figure. 

 ITie accuracy of the instrument evidently 



depends upon the perfect equality of its 



two opposite ends. To ascertain, if it be 



accurately constructed, Mr. Ritchie directs 



us to place it between two steady flames, and 



move it nearer the one or the other till the 



liquid in the tube remains stationary, at the 



division of the scale at which it fonnerly 



stood. Turn it half round without altering 



its distances from the flames, and if the 



liquid remains stationary at the same division, 



the instrument is correct. To show the 



extreme delicacy of the instrument, he says, 



that if placed opposite a single candle, it will 



be sensibly aflfected at the distance of 10, 



20, or 30 feet, provided it be of sufficient 



diameter, whilst it will not be sensibly acted 



upon at the same distance by a mass of 



heated iron affording twenty times the quan- 

 tity of heat. 



Our author then proceeds as follows: — 



" Place the instrument between any number of 

 steady lights whose intensities are known, as 

 for example, between four wax candles oppo- 

 site one end, and one candle opposite the other, and move the pho- 

 tometer till the fluid remain stationary 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 consuming known quantities of gas in equal times, and the 

 photometer be placed between them, so that the effect upon the air 

 in each chamber shall be the same ; it will be found that the quan- 

 tities of gas consumed by each, will be exactly proportional to the 

 squares of the distances of their respective flames from the ends 

 of the photometer." 



" This instrument seems well adapted for determining the re- 

 lative 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 squar- 

 ing their distances from the adjacent ends of the instrument ; as-^ 



