THE STEAM-ENGINE. 



409 



plate R (fig. 2) is moved from the one tube to the other, and at the same time, 

 as we have already stated, the condensing-pipe is brought to play on V. While, 

 therefore, a vacuum is being formed in V by condensation, the steam, flowing 

 through T 7 , blows out the air through B x , as already described in the other ves- 

 sel V ; and while the air in S is rushing up through A into V, followed by the 

 water raised in S by the atmospheric pressure, the vessel V is being filled with 

 steam, and the air is completely expelled from it. 



The communication between T and the boiler is now again opened, and the 

 communication between T 7 and the boiler closed by moving the regulator R 

 (fig. 2) from the tube T to T 7 ; at the same time the condensing pipe is re- 

 moved from over V, and brought to play upon V. While the steam once more 

 expels the air from V through B, a vacuum is formed by condensation in V 7 , 

 into which the water in S rushes through the valve A 7 . In the meantime V is 

 again filled with steam. The communication between T and the boiler is now 

 closed, and that between T 7 and the boiler is opened, and the condensing pipe 

 removed from V, and brought to play on V. While the steam from the boiler 

 forces the water in V through B 7 into the force-pipe F, a vacuum is being 

 produced in V, into which water is raised by the atmospheric pressure. 



Thus each of the vessels V V 7 is alternately filled from S, and the water 

 thence forced into F. The same steam which forces the water from the ves- 

 sels into F, having done its duty, is condensed, and brings up the water from 

 S, by giving effect to the atmospheric pressure. 



During this process, two alternate motions or adjustments must be constantly 

 made ; the communication between T and the boiler must be opened, and that 

 between T 7 and the boiler closed, which is done by one motion of the regula- 

 tor. The condensing pipe at the same time must be brought from V to play 

 on V 7 , which is done by the lever placed upon it. Again the communication 

 between T 7 and the boiler is to be opened, and that between T and the boiler 

 closed ; this is done by moving back the regulator. The condensing-pipe is 

 brought from V 7 to V by moving back the other lever, and so on alternately. 



In order duly to appreciate the value of improvements, it is necessary first 

 to perceive the defects which these improvements are designed to remove. 

 Savery's steam-engine, considering how little was known of the value and 

 properties of steam, and how low the general standard of mechanical knowl- 

 edge was in his day, is certainly highly creditable to his genius. Neverthe- 

 less it had very considerable defects, and was finally found to be inefficient for 

 the most important purposes to which he proposed applying it. 



At the time of this invention, the mines in England had greatly increased in 

 depth, and the process of draining them had become both expensive and diffi- 

 cult ; so much so, that it was found in many instances that their produce did. 

 not cover the cost of working them. The drainage of these mines was the 

 most important purpose to which Savery proposed to apply his steam-engine. 



It has been already stated that the pressure of the atmosphere amounts to 

 about fifteen pounds on every square inch. Now, a column of water, whose 

 base is one square inch, and whose height is thirty-four feet, weighs about 

 fifteen pounds. If we suppose that a perfect vacuum were produced in the 

 steam-vessels V V 7 (fig. 3), by condensation, the atmospheric pressure \vould 

 fail to force up the water, if the height of the top of these vessels above the 

 water to be raised exceeded thirty-four feet. It is plain, therefore, that the 

 engine cannot be more than thirty-four feet above the water which it is in- 

 tended to elevate. But in fact it cannot be so much ; for the vacuum produced 

 in the steam-vessels V V 7 is never perfect. Water, when not submitted to the 

 pressure of the atmosphere, will vaporize at a very low temperature, as we 

 shall hereafter explain ; and it was found that a vapor possessing a considera- 



