M. Faraday on Fumigation. 95 



be fumigated in the most unexceptionable manner, and the means 

 employed were therefore applied to an extent probably far beyond 

 that requisite to the destruction of any miasmata that might be 

 within it. The proportion of chlorine evolved to the size and 

 surface of the building may be considered therefore as sufficient 

 for a case of the most excessive kind ; and, though the limits are 

 guessed at rather than judged of by any well-founded rule, yet I 

 should consider from one-half to one-fourth of the chlorine as 

 quite sufficient for any of the usual cases where fumigation is 

 required. 



Art. IX. On the General Nature and Advantages of Wheels 

 and Springs for Carriages, the Draft of Cattle, and the 

 Form of Roads. By Davies Gilbert, Esq. F.R.S,, 8fc. 



Taking wheels completely in the abstract, they must be consi- 

 dered as answering two different purposes. 



First, they transfer the friction which would take place between 

 a sliding body and the comparatively rough uneven surface over 

 which it slides, to the smooth oiled peripheries of the axis and box, 

 where the absolute quantity of the friction as opposing resistance 

 is also diminished by leverage, in the proportion of the wheel to 

 that of the axis. 



Secondly, they procure mechanical advantage for overcoming 

 obstacles in proportion to the square roots of their diameters 

 when the obstacles are relatively small, by increasing the 

 time in that ratio, during which the wheel ascends : and they 

 pass over small transverse ruts, hollows, or pits, with an absolute 

 advantage of not sinking, proportionate to their diameters, and 

 with a mechanical one as before, proportionate to the square roots 

 of their diameters. 



Consequently, wheels thus considered cannot be too large : in 

 practice, however, they are limited by weight, by expense, and by 

 convenience. 



With reference to the preservation of roads, wheels should be 



