136 LECTURES TO SCIENCE TEACHERS. 



girders. There were also two inner tie-rods, one at the 

 top, and one at the bottom, which, passing through all the 

 chambers and through certain washers, served at the same 

 time as tie-rods, and by the annular spaces between the rods 

 and the insides of the washers as a steam-pipe at the top 

 and a feed-pipe at the bottom. The plates were not above 

 J of an inch thick, and the pressure of steam was, I believe, 

 as much as 80 Ibs. , and those boilers were the means whereby 

 Hancock was enabled upwards of thirty years ago to drive his 

 carriages through the streets of London, and to take passengers 

 from the bank to Paddington and to Whitechapel regularly 

 amongst the omnibus traffic without hindrance of any kind 

 or description. There were various meritorious points in that 

 steam-carriage, which I should like to go into if time permitted. 

 Up to the present it has not only not been surpassed, 

 but I am sorry to say that particular branch of engineering 

 has gone backwards, and up to the present time Hancock's 

 carriage has not been equalled. It was noiseless, smokeless, 

 sightly, easily steered, light and powerful, necessary qualities 

 all for street navigation and for avoiding the alarming of 

 horses. Hancock attended to all these points, while the 

 engineer of the present day, although he may make a very 

 good engine, does not appear to consider that he has to 

 make one which is to go about amongst horse traffic. 



With respect to the proportion of boilers, in the table that 

 I exhibit here of the engines tried at the Royal Agricul- 

 tural Society, I may tell you that the proportion of absorbing 

 surface of the engines varied from 16 superficial feet per 

 cubic foot of water evaporated per hour, to 37 superficial 

 feet. With the 16 feet there was obtained '651 of the total 

 heat effect of the coal, and the gases escaped at 775. With 

 the 37 feet super there was obtained '776 of the total effect 

 of coal, and the gases escaped at 500. I may say in the 

 face of such discrepancies in proportion as those, it is difficult 

 to fix on any rule, but the ordinary rule is from 12 to 15 

 superficial feet of heating surface for each cubic foot of water 

 evaporated per hour, while the surface of grate where there 

 is no forced draft is from -^th to -g^th of the heating 

 surface. 



Before leaving the subject of boilers I should like to call 

 your attention to the solar boiler which is being worked at 

 Tours ; an account of this boiler is to be found in the Revue 



