Prof. Gerling^s Description of two Letter-scales. 113 



the iodide in small holes. If this occur the bath can be restored 

 by nearly, but not quite, precipitating the silver with a solution 

 of chloride of magnesium, and then filtering. 



One of the most important things to be attended to is the 

 necessity of preserving the plates where they are perfectly free 

 from any light. It will be evident to all, that anything short of 

 absolute darkness, when the sensitive surface is exposed' to its 

 action for day after day, and perhaps week after week, must be 

 fatal to its subsequent cleanliness. The necessity for protecting 

 the plates from any deleterious gases, ammonia, for instance, is 

 too obvious to require comment. 



XVI. Description of two Letter-scales. 

 By Prof. Gerling of Marburg^. 

 [With a Plate.] 



THE introduction of postage- stamps renders it desirable that 

 private individuals should possess the means of weighing 

 letters with accuracy and facility. It will perhaps be permitted 

 me, in addition to the letter-scales already in use, to describe 

 two others which a yearns practice has proved correct, and which 

 on account of their simplicity are comparatively cheap. 



A side view of the first instrument is given in Plate I. fig. 1. 

 It consists of the unmoveable slab of wood A, and the four small 

 wooden pillars B, which latter are united by the thin steel axes C. 

 The moveable portions consist, in the first place, of four equal 

 bent levers D, C, E of sheet metal. Through the ends of these 

 levers pass the four axes DD and EE. These axes carry the two 

 strips of sheet metal DD, a piece of metallic gauze being spread 

 across from one strip to the other to form the table on which 

 the letter is placed. Below are two similar strips of metal, EE, 

 which serve as a counter- weight to the letter placed above. The 

 strips EE which remain horizontal, move more or less to the 

 right according as the weight placed above is greater or less. 

 The wire ^ is a fixed index, and the divisions which answer to a 

 quarter of an ounce, half an ounce, an ounce, and so forth, are 

 first determined empirically and marked along the strip EE. 



The second balance, fig. 2, consists simply of a glass tube filled 

 with water, into which another smaller tube passes and sinks to 

 a certain depth ; the latter tube is furnished with a little plat- 

 form on the top, on which the letter is weighed. The internal 

 tube sinks when a weight is placed above, and the depth to which 

 it sinks for certain weights is first empirically determined ; these 

 depths are marked upon the surface of the tube, and are the in- 

 dices by which the weight of the letter placed above is given. 

 * Communicated by the Author. 



Phil Mag, S. 4. Vol. 8. No, 50. Aug. 1854. I 



