2Q PREFACE. 



In the lectures on the steam-engine, I used large sectional 

 models as illustrations. In lieu of these, the present vol- 

 umes are illustrated with an extensive collection of plans 

 and sections of steam-engines and their various parts, made 

 on a scale as large as the size of these pages admitted. 

 Among these, may be mentioned, as more especially de- 

 serving of attention, the series of eight large drawings of 

 the locomotive-engines of Messrs. " Stephenson and Com- 

 pany." 



It may be proper to observe here, that, as these discourses 

 were designed for the use of the general reader, the prac- 

 tice I have found beneficial in my lectures, of using round 

 numbers in preference to the exact numerical value, has 

 been persevered in. Round numbers have the advantage 

 of being easily impressed on the memory ; and for the pur- 

 poses of the readers for whose use these volumes are in- 

 tended, they have all the necessary utility. Thus, for ex- 

 ample, the distance of the earth from the sun is generally 

 stated as a hundred millions of miles. This is easily re- 

 membered. Nor is it of any real importance for the objects 

 of general information, that the real distance is more ex- 



o ' 



actly ninety-five millions of miles. Again, the pressure of 

 the atmosphere is a varying quantity, changing not only 

 daily and hourly everywhere, but even at the same time dif- 

 fering in different places. It would be impossible to fix in 

 the memory its average values at each season of the year, 

 and at different places ; but it is very useful and satisfac- 

 tory to know that it may be assumed generally to be at the 

 rate of about fifteen pounds on every square inch of surface 

 exposed to its action. 



\ These volumes have been designed for general informa- 

 i tion and amusement, rather than for the purposes of refer- 



