rebuilding London Bridge. 35 



In the case of the iron arch of 600 feet span, designed by 

 Mr. Telford, for the same site, which was to have cost only 

 262,2897., it was thought right, that a series of questions should 

 be submitted to the scientific and practical men of the age, accom- 

 panied by drawings of that proposed structure, and their answers 

 have been of considerable advantage, in extending a knowledge of 

 the science of bridge building. It would be a great public benefit 

 if the same proceeding were adopted in this case ; but the question 

 and data ought to be corrected, if not prepared, by some one 

 who knows both what will be useful to the practical professor, and 

 what will be amusing to the mathematician. 



Art . IV. Remarks on the Deposition of Dew. By George 

 Harvey, M.G.S., #c.#c. 



Du Fay mentions an interesting example of the influence of 

 metals in retarding the deposition of dew. He found, that a 

 watch glass resting on a silver plate, and surrounded with a ferrule 

 of the same metal, had a space round its border five or six lines 

 wide, perfectly dry, and towards which the drops regularly de- 

 creased in magnitude. In performing the experiment, however, 

 I found, that not only a dry zone surrounded the edge of the glass, 

 but a circular space, equally dry, existed in the middle of its sur- 

 face, together with two narrow zones of dew, composed of exceed- 

 ingly fine particles, surrounding the borders of the middle zone, 

 and which last was covered with drops of a larger size. 



At the time the experiment was performed, the whole night was 

 devoted to some general inquiries relative to dew ; so that ample 

 opportunity was afforded of observing its first deposition, and also 

 the gradual steps by which it successively accumulated. 



At sunset, on the 15th of May, two watch glasses, of equal di- 

 mensions, were placed, with their concave sides upwards, on a 

 plate of highly polished block tin, one of them being surrounded 

 with a ferrule of the same metal, of the same diameter as the glass, 

 and a depth equal to its versed sine. 



It was not until half an hour after sunset, that the temperature 

 D2 



