1875] WRITINGS OF JOSEPH HENRY, 489 



ings are taken on one side of the scale, and then the photo- 

 meter is reversed and as many taken from the opposite side. 

 In this way the mean of sixty readings, twenty on each scale, 

 furnishes the data on which the character of the oil princi- 

 pally rests. As a means of simultaneously weighing the 

 candles for checking the effects of their irregular burning, 

 three balances are provided, each of which bears one of the 

 candles in a socket supported by a metallic link, through 

 which the scale-beam passes and is attached to the hook of 

 the scale-pan below. 



On the opposite scale-pan a series of grain weights are 

 placed, which can be taken off by a pair of pincers without 

 disturbing the equilibrium of the scale; the interval of time 

 during which a given grain weight is burned is marked 

 by a watch. If the interval is equal to two grains for each 

 minute, the candle is burning at its normal rate; if not, a 

 correction is made by simple proportion, which is applied to 

 the measurement previously obtained. 



The lamps containing the oil for trial are lighted and 

 trimmed in an adjoining apartment. They are introduced 

 into the dark room through a window closed with a sliding 

 shutter. In order to prevent an overflow of oil at the burner 

 by the oscillation of the liquid in the reservoir by the agita- 

 tion of transfer, each lamp is placed on a small carriage 

 moving on a railway passing through the window, which 

 enables the lamp to be placed in its position with rapidity, 

 and without the slightest disturbance of the equilibrium of 

 the oil. 



The temperature of the room is also noted, and as far as 

 possible it is kept at a heat of not far from 70°. For this 

 purpose — during warm weather, the inspection may be made 

 at night. 



For reading the divisions on the scales in the dark room, 

 a mirror is employed to throw the light of the lamp under 

 inspection on the graduation. 



To exclude all extraneous light, the three candles and the 

 lamp to be tested — are each surrounded by a cylindrical 

 sheet-iron screen painted black, through which a hole (a little 

 larger than the flame) allows the light to pass along the scale 



