Processes in the Usef?d Arts. 341 



oil and coal-gas, this instrument will give a pretty good approximation to 

 their illuminating powers, but unless the colour ofthejlames be exactly the 

 same, neither this method nor that of shadows can pretend to more than a 

 mere approximation. I may here remark, that the results of my photo- 

 meter, or of any other instrument founded on the expansion of air by its 

 combination with light, will always be unfavourable to the illuminating 

 powers of oil-gas compared with coal-gas, as it takes no cognizance of the 

 Jine white colour of the flame of the former compared with the more 

 dusky colour of the latter. When the oil-gas is of a superior quality, and 

 the coal-gas not well purified, the results of this photometer, or that of Pro- 

 fessor Leslie, will be extremely wide of the truth. The indications of 

 this instrument may give the quantities of light as one to three, whilst the 

 illuminating power of the oil-gas is at least five or six times greater than 

 that of the coal-gas." — See p. 321 of this Number. 



2. M. Ducom's Cylindrical Artificial Horizon. 



This ingenious instrument, which has already been put to the test of 

 experiment by two able astronomers, Professor SimonofF and Baron Zach, 

 has not yet been described by the inventor, but the general principle of 

 the instrument may be deduced from two long dissertations upon it by 

 Baron Zach. 



The instrument consists of two parts, one of which is a copper disk of six 

 inches in diameter, with three feet. The second part is a cylindrical cover 

 or drum, which performs the part of the glass roof in the common hori- 

 zons for sheltering the fluid from the action of the wind. From the 

 middle of the first part, or copper disk, there rises a hollow cylinder of 

 white iron 41 inches high, and 2| inches in diameter. Upon this cylin- 

 der, which is open at top, there is placed a small round disk of white 

 iron, (or of boxwood, when mercury is used,) which goes into the top of 

 the cylinder, but is prevented from descending by a ledge on which it 

 resis. This disk contains the mercury, wine, or prepared syrup which is 

 employed. These cylinders are adjusted in such a manner that the sur- 

 face of the fluid is exactly 2\ inches above the first disk. On the copper 

 disk are fixed two brackets, to which is fastened the cylindrical roof or drum. 

 This drum, which is made of white iron, is six inches in diameter and 2fc 

 wide, and is so placed that its centre is in the surface of the fluid in the 

 round disk. In the middle of the width of the drum, there are two bands 

 of white iron, perforated by two circular openings diametrically opposite 

 to one another, and an inch in diameter, the one for letting in the inci- 

 dent rays, and the other for letting out the reflected ones. They have a 

 circular motion by a rack and pinion on the surface of the drum, for the 

 purpose of being adjusted to the height of the sun or the star. 



When there is not much agitation in the air, two small funnels or 

 truncated cones are placed in the small tubes in the circular apertures, 

 and these have the effect of protecting the fluid surface from every agita- 

 tion. When the wind is considerable, the funnels are kept on, and a small 

 glass with parallel faces is placed at the end, by which means the incident 

 rays arc admitted ; but if the wind is very high, the funnels arc taken off; 



