274< Itctrospective Criticism. 



rooms through gratings placed in the floor. The same effect must take place 

 in this apparatus of Mr. Pcnn's; and I feel convinced, by practical experi- 

 ence, that it is impossible to produce such a high temperature and so uniform 

 a heat by enclosed pipes, as by those which are freely exposed in the building. 

 Another advantage claimed for this apparatus appears to me more than 

 doubtful. It is stated that, by throwing water into the tunnel, any degree of 

 moisture may be imparted to the air ; but on this subject I would remark 

 that air absorbs moisture only in proportion to its heat; and, as in this case it 

 is heated by the pipes after it has passed through the tunnel, it does not appear 

 to me that it can ever be saturated by this means. 



The sixth and last claim which you state this plan has to public notice is, 

 that, " in the atmosphere of London, where the air is charged with soot and 

 smoke, Mr. Penn's improvement will admit of forming a green-house or stove, 

 with much purer air than can be obtained by admitting the external atmo- 

 sphere according to the usual means of ventilation, which will not only be 

 better for plants, but for persons going in to examine them." This passage I 

 do not profess to understand, and therefore can offer no remarks upon it; for 

 it appears to me, that, as Mr. Penn draws the air required for ventilation from 

 the external air, that which he thus uses must be the same as is obtained 

 by other people. 



With respect to the Conical Boiler of Mr. Rogers, I would also offer a few 

 remarks. Your correspondent, Mr. Beaton, states that when this boiler was first 

 used it was in the form of a vertical cylinder, " which did not answer perfectly^ 

 wasting much heat : " but, in another part of his letter, he states, " there is 

 great inconvenience experienced by the formation of steam, and he therefore 

 thinks it will be better more of a cyHndrical form," that is to say, of the very 

 form which experience has shown wastes much heat. It would, therefore, 

 appear, that either Scylla or Charybdis must be our fate, when we use this boiler- 



Mr. Rogers has himself, with the greatest fairness, noted several of the 

 inconveniences which attend the use of his boiler, and which have induced him 

 unwillingly to relinquish the old form for that proposed by Mr. Shewin» 

 These inconveniences, he states, were, the hability to derangement, and da- 

 mage to the grating, as also the rapid wear of the copper boiler. If then, 

 on the evidence of the avowed advocates for this kind of boiler, we find 

 that, in order to avoid a paramount evil, we must adopt a shape which wastes 

 much heat, that is also liable to get out of order, and that has sometimes, 

 been found " corroded in a few months by the sulphur disengaged from 

 the coke," I am utterly at a loss to know why it should be so highly ex- 

 tolled, or to discover in what its merit consists. Of the improvement 

 suggestetl by Mr. Shewin nothing certain is yet known, for it appears that at 

 present it is quite in a crude state, and has not been sufficiently tried, to 

 ascertain whether some greater inconvenience may not result from its use, 

 than those which it is the object to avoid. 



I cannot subscribe to the doctrine, that the economy of fuel must be 

 greater in this than in other boilers. The theory of combustion is now toa 

 well known to lead us astray in this matter. Slow combustion and a small 

 degree of heat, are not the most economical ; for in this case it is known,, 

 that the carbon of the fuel is changed into carbonic oxide, which contains a 

 considerable body of latent heat, whereas a rapid combustion changes it 

 into carbonic acid, which is the most extreme change that fuel can undergo, 

 and by which alone its con)plete combustion is effected. Another objection 

 is, that it is only the internal surface of the boiler which is exposed to the fire. 

 I have, I believe, seen every kind of boiler that has yet been used for hot 

 water, and I am of opinion that no shape exceeds in efficiency the horse- 

 shoe, or saddle-boiler. It has been proved to be equally suitable for large 

 or small apparatus, for I have seen as much as 1000 ft. of 4-inch pipe, heated 

 well by one boiler, and some have come under my notice, which were heated 

 by Mr. Fowler of Temple Bar, and appeared to me to be as near to perfec- 

 tion as could well be imagined. I have no doubt, however, that a great 



