44; Retrospective Criticism. 



conical boilers, has been that the public should have the benefit of them, such 

 as it may be, in the most efficient form, I think it but fair that all who have 

 lent themselves to the carrying of my plans into execution should share 

 equally the advantage, such as it may be, of my recommendation. I find that 

 Messrs. Lankester and Co., of Southampton, to whom I communicated my 

 suggestions as long ago as 1836, have carried them into execution very ex- 

 tensively, and with great success. They continue to employ copper boilers, 

 not having experienced the inconvenience which I found in the use of that 

 metal ; and, though I confess I have some misgivings as to its durability, my 

 single experiment cannot be considered as conclusive against it. I have not 

 had the advantage of seeing any of the apparatus erected by them at work, 

 but the numerous instances of their successful application which they have 

 handed to me, and the general efficiency v/ith which all their undertakings are 

 conducted, leave me no room to doubt that they would execute satisfactorily 

 any work which may be intrusted to them. 



In taking leave of my now almost jaded hobby, I would once more suggest 

 that it is essential that the conical boilers should be set, or the setting ac- 

 tually superintended, by some one who thoroughly understands the principle 

 upon which they are to act, or failure and disappointment will probably ensue. 

 I should also mention that more extended experience has satisfied me, that 

 the reverberatory mode of setting, in which the draft descends around the 

 outside of the boiler before it enters the chimnej', is invariably the best : 

 boilers so set require less attention and less skill to manage them than those 

 fixed as my own, and some of the earlier ones. 



In conclusion, I must disclaim any merit for any thing more than the mere 

 contrivance of the boiler itself; for all the calculations necessary for its ap- 

 plication to produce any given temperature, both as to the size of the boiler 

 itself, and the pipes which it is capable of heating, I am indebted to Mr. 

 Charles Wood's most valuable treatise. My own experiments on these 

 points have been few and very roughly conducted ; and the results I deduced 

 from them, though perhaps not very erroneous, were merely empirical, and 

 applicable only to similar or not very dissimilar cases. I have relied entirely 

 upon his experiments and calculations, wherever I have had occasion to advise 

 the quantity of pipe which should be employed for any specific purpose ; and 

 I cannot neglect this opportunity of acknowledging his very obliging personal 

 communications on the subject, whenever I have taken the liberty of con- 

 ferring : nevertheless he is no way responsible for any blunders which I may 

 have committed. The conical boilers are now in the hands of a great number 

 of persons, and I am happy to find that general testimony confirms the 

 favourable opinion I entertained of them ; nevertheless, I am far from be- 

 lieving that other boilers of different form might not be made equally or more 

 efficient, if constructed with the same attention to the purposes to which they 

 are to be applied. — John Rogers. Sevenoaks, Dec. 4. 1 840. 



Semicylindrical Draining-Tiles. — In the Gardener'' s Magazine for 1840, p. 535., 

 I am somewhat surprised to see it stated by you, that the seniicylindrical 

 draining-tiles are placed with the open side downwards, resting upon fiat 

 tiles. Now it is obvious that the contrary is the proper way of placing them ; 

 because by resting them upon the convex side, with the concave side upper- 

 most, the whole force of the current, be it large or small, will be collected 

 into one body along the centre of the drain, and by collecting all the force 

 into the smallest possible space, it will generally be found to keep itself clear 

 of sand or any kind of dirt that may, from time to time, be washed into it. 



The only objection which I can discover, against placing the tiles with the 

 open side uppermost, is, that the covers might be liable to be broken by the 

 passing over them of loaded wheelbarrows, &c. This objection is, however, 

 of such trivial importance, that it could easily be obviated by placing the 

 drains sufficiently deep, that the covers might be buried 10 or 12 inches be- 

 neath the surface of the walks. Although this depth would be found quite 



