PRIME-MOVERS. 371 



I wish to say a few words which will bring before you the changes 

 that have been made within a very few years in the construction of 

 steam-engines : when I was an apprentice the ordinary working 

 pressure of steam, except in the double cylinder engine, was only 3 Ibs. 

 above atmosphere, and in those days there was in a marine boiler 

 more pressure on its bottom when the steam was down due to the 

 mere head of water in the boiler than there was pressure on the 

 top when the steam was up due to the force of the steam, whereas 

 now condensing marine engines work commonly at 70 Ibs. ; and there 

 is a boat under trial where the steam is, I believe, as high as 400 Ibs. 



To those who are curious on the subject I would recommend a 

 perusal of two Blue Books, one being the evidence taken before a 

 Parliamentary Commission in 1817, and the other before a Parlia- 

 mentary Committee in 1839, they will find there the weight of 

 evidence to be that the only use of high pressure steam is to dis- 

 pense with condensing water, and that as a steamboat must always 

 have plenty of condensing water in its neighbourhood, no engineer, 

 knowing his business, would suggest high pressure for a marine engine. 

 I have before me a model of a pair of engines which, although 

 they weie made not so very long ago (for I saw them put into 

 the ship), have nevertheless an historical interest. This model shows 

 Maudslay's engines of the Great Western, the first steamer built 

 for the purpose of crossing the Atlantic ; I think I am right in 

 saying that 7 Ibs. steam was the pressure employed in that vessel, and 

 that, in order to extract the brine from the boiler, it was necessary to 

 use pumps, as the pressure of the steam was not sufficient to expel 

 the brine and to deliver it against the pressure of the sea. 



Time does not permit of nry touching upon the various improve- 

 ments in boilers, condensers, expansive arrangements, and other 

 matters which have gradually been introduced into our best engines 

 for land and for ocean purposes. I have hung upon the wall a 

 rough diagram, showing a pair of oscillating engines as applied to 

 driving a paddle steamer, and another showing a pair of inverted 

 compound cylinder engines to drive a screw propeller, a model of 

 such a pair of engines, with surface condensers and all modern 

 appliances, (being Messrs. Rennie's engines of the P. and O. Com- 



