STEAM 



STEAM-DIGGER 



699 



When water is boiled in an open vessel neither 

 the temperature of the water nor that of the steam 

 rising from it ever rises higher than 212, however 

 hot the fire ; the heat as it enters is carried off in a 

 latent state in the steam. But under pressure the 

 temperature of both can be raised to any degree. 

 If, when the water and steam in a ( above ) came to 

 212, the application of heat were still continued, 

 more steam would continue to rise, and, the pressure 

 on the under side of the piston being now greater 

 than that of the air above it, the piston would begin 

 to ascend ; but suppose it held in the same position 

 by force, the upward pressure of the steam would be 

 found rapidly to increase until it would soon 

 require a weight of 14'7 Ib. per square inch to keep 

 it down, showing that the pressure of the steam 

 was now equal to twice that of the atmosphere, or 

 to 29-4 Ib. per square inch. If at this point the 

 temperature of the water and steam were examined, 

 it would be found to be very nearly 250 F. When 

 the absolute pressure of the steam reached 50 Ib. 

 its temperature would be 281; at 100 Ib., 328; 

 at 150 Ib., 360, and so on. 



From the numerous experiments made on this 

 subject some very important general conclusions 

 may be drawn. Of these one which will be 

 evident from the figures just given is that the 

 pressure of steam increases at a far higher rate 

 than the temperature (doubling the temperature 

 increases the pressure nearly S3 times), which shows 

 the extreme danger of continuing to apply heat to 

 a vessel from which the steam is not allowed to 

 escape. The bursting force would soon become 

 such as no vessel could resist. Another important 

 conclusion is that for every temperature there is a 

 corresponding density of steam produced. This 

 steam contains a definite amount of latent heat, 

 and exerts a certain uniform pressure on every side 

 of any vessel in which it may be contained. The 

 following table shows the relation between these 

 values for steam of several different temperatures : 



T. p. 



82* 0-085 



104' 1-08 



158* 4-61 



tie 14-7 



248' 28-83 



*W 604 



858- 145-8 



401* 2603 



H. 



1091-8 

 1113-7 

 1130-1 

 1146-0 

 1157-5 

 1171-2 

 11904 

 1204-1 



V. v. 



3390-0 211,536 



:,1 - 18,519 



80-02 4,993 



26-36 1,645 



14-0 874 



6-992 436 



8-057 191 



1-838 115 



T, Temperature In degrees Fahrenheit ; p, absolute pressure In 

 pounds per square inch of the steam at that temperature. 

 H, Total heat of the vapour above 32 F. at that temperature 

 (according to Regnault's experimental in thermal units. A 

 thermal unit 772 foot-pounds) Is the quantity of heat which 

 will raise 1 Ib. of water 1* F. at or near it temperature of 

 greatest density, 39'!' F. The specific heat of water increases 

 lowly as the temperature rises, so that 1 thermal unit will 

 not raise 1 Ib. of water quite no much as 1* at high tempera- 

 tures. V, Volume in cubic feet occupied by 1 Ib. of steam ; 

 v, number of times which volume of steam exceeds that of 

 aarne weight of water. 



The relations between temperature and pressure 

 in the foregoing table apply only so long as the 

 Bteam is in contact with the water from which it is 

 generated. Once away from the water its tempera- 

 ture may be raised without altering its pressure. 

 Steam which has received additional heat in this 

 way is called superheated steam. It approximates 

 to the condition of a perfect ga, and therefore 

 follows nearly what is known as Boyle's or Mari- 

 otte's Law, its volume varying always inversely as 

 it* pressure. By this law steam which occupied 1 

 cubic foot at 20 Ib. absolute pressure would occupy 

 4 cubic feet at 5 Ib., and halt a cubic foot at 40 Ib. 

 absolute pressure. But steam, as commonly used 

 in the steam-engine, is not superheated, but used 

 nnder the conditions given in the table. It is then 

 called saturated steam, and differs sensibly from 

 the condition of a perfect gas. If the pressure (p) 

 be given in pounds per square inch, and the pro- 



duct (pv) of pressure and volume in foot-pounds, 

 then the formula, log. (pv) = 4'675 + -061 log. p, 

 gives results accurate enough at all ordinary pres- 

 sures, and can be very easily applied. The volume, 

 instead of increasing inversely as the pressure, in- 

 creases less rapidly ; the difference, though not 

 very great, is so large that it has to be taken into 

 account in all calculations as to the efficiency and 

 behaviour of steam in a steam-engine. 



It might naturally be expected that it would 

 take much more heat or fuel to convert a pound 

 of water into steam at a higher than at a lower 

 temperature and pressure. In reality, however, 

 the difference is very slight. Referring back to 

 the table it will be seen that it requires 1146 '6 

 units of heat to raise a pound of water from 32 to 

 212, and evaporate it at that temperature ; of 

 these 180 are expended in raising the temperature, 

 while 1146-6 - 180, or 966 -6 units, become latent in 

 the steam. It only requires 1171-2 unite, however 

 (261 sensible and 910-2 latent), to raise the water 

 to 293, and evaporate it at that temperature ; for 

 the latent heat falls nearly as fast as the sensible 

 heat rises. The additional heat required is thus 

 only a little over 2 per cent., while the pressure 

 which is, co-tens paribus, a measure of the work the 

 steam will do is more than quadrupled. In this 

 way a large increase of power in any engine may 

 be obtained by a small additional expenditure of 

 fuel, and consequently steam of a high pressure is 

 now being used for all purposes, its economy and 

 advantages being fully recognised by engineers. 

 It was thought for a long time that the total heat 

 of steam i.f. the sum of the sensible and 

 latent heats was constant at all temperatures; 

 but this is not strictly the case, although the table 

 shows that the difference for ordinary ranges of 

 pressure is but trifling. See HEAT, and GAS. 



Steam-carriage. See TRACTION-ENGINE. 



Steam-crane. See CRANE. 



Steam-digger. The successful application 

 of steam to the ' digging ' of the soil can hardly he 

 said to date further back than 1880. In that year 

 at the Koyal English Show at Carlisle Messrs 

 M'Laren of Leeds exhibited the Darby Digger, for 

 which they received the society's special silver 

 medal. This digger was the invention of Mr T. C. 

 Darby of Pleshey Lodge, Chelmsford, a gentleman- 

 farmer, who expended large sums of money and much 

 anxious labour upon the perfecting of this most use- 

 ful implement. The problem of steam -digging was 

 now successfully solved, and it is surprising that the 

 system has not been more largely adopted. The 

 action of the digger in the soil is quite different 

 from that of the plough. It is much more bene- 

 ficial to the soil from almost all points of view. 

 Not only is there a saving in power, but the quality 

 of the work done is far superior to that accom- 

 plished by the plough. The steam-digger indeed 

 imitates closely digging by hand, and hand-digging 

 is the most perfect of all methods of tillage. By 

 the process of digging the soil is much more 

 thoroughly pulverised than by ploughing. The 

 digging-forks of the steam-digger tear up the soil 

 and toss it over in forkfuls in a manner which 

 leaves both the subsoil and surface-soil more open 

 than is the case in ploughing. The action of the 

 plough in cutting the furrow from the subsoil tends 

 to the formation of a ' pan ' on the top of the sub- 

 soil. The digger ' tears ' instead of ' cuts ' the sur- 

 face-soil from the subsoil, and this tearing action 

 tends to loosen the latter. Most farmers who have 

 tried both steam-ploughing and steam-digging 

 much prefer the fatter, not only because they 

 believe that from digging the crops are better and 

 the weeds fewer, but also because the digging 

 appliances are much more simple, and involve less 



