1841.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



411 



two reLisons the effect due to this alteration of temperature vanishes, 

 in the firbt pfjce Mr. M. will surely not deny that if the part of the 

 steara pipe within the smoke box and the cylinders were tilled 0th 

 water it would speedily be evaporated, which must be in consequence 

 of heat imparted, and it is certainly very c iJent that at least part of 

 that heat must enter the steam; and Mr. M. will find that although 

 the diminution of temperature due to expansion (alone considered) 

 becomes greater with the expansion, so also does both, the surfaces of 

 the cylinder and steam pipe, and the time for which the steam is ex- 

 posed to the heat of the smoke box ; again in the second phice, if Mr. 

 M. had introduced into his laborious calculation?, to jirove that he was 

 right in objecting to the omission of the cifect of the steam during the 

 part of the stroke from the opening of the eduction port to the termi- 

 Dation of the stroke, the remainder of the ertVct of ihis diminution of 

 temperature due :o expansion, he would have found that the one so 

 balanced the other that the correction to be introduced though appre- 

 ciable, is of analogous importance «ith f^ of one per cent. 



The remainder of Mr. M.'s last letter is taken up in correcting the 

 values of (a' and 6) that I calculated for the four particular cases, 

 which presents a beautiful illustration of a person, in his attempt to 

 have the ]>'easure of helping another over a ditch, steppiug into it 

 himself; when Mr. M. first stated that the equations wliich give the 

 values of (a' and b) are themselves correct, but that the numerical 

 values which I calculated were incorrect, I thought lu meant that I 

 bad made some numerical mistake, and therefore did not trouble my- 

 self about it, but I now see that he imagines I have made a radical 

 error in applying the formulEe, by making incorrect substitutions for 

 the known quantities. Had the formula; been made on the hypothesis 

 that Mr. M. thinks they were made on, they would (instead of being 

 as Mr. M. thinks quite correct) have been altogether wrong ; Mr. M. 

 seems to think that the radius of the eccentric is a function of the 

 cover of the slide, whereas such I never hitended it to b", and such it 

 is not which Mr. M. might liave very easily discovered by looking at 

 the investigation of these formufe. The stroke of the valve being in 

 DO degree dependent on the cover of the slide does not at all effect 

 the size of the port for the admission of the steam. Mr. M. will hence 

 find it necessary to recalculate his values of (a' and 6), and make the 

 alterations thereby caused on his laborious calculations with reference 

 to the omission of the effect due to part of the stroke from the opening 

 of the eduction port to the termination of the stroke. 



I %vill now conclude this letter by remarking that so far as economy 

 of fuel is concerned, so far as the advantage that an engine working 

 expansively has over one working with full pressure is concerned, we 

 may neglect taking into account all things that equally affect both, 

 ■which 1 would recommend Mr. M. to consider in estimating the value 

 of some of his objections. 



And I remain. Sir, 



Your obedient servant, 



J. G. Lawrie. 



Cartsdyke Foundry, Greenock, 

 Novemler \o, 1841. 



f 



EVAPORATION OF NVATER. 



Sir — Having obtained a patent for certain improvements in evapor- 

 ation, I take the liberty of sending you a description thereof, and also 

 an account of the result I have obtained from the appaiatus; should 

 you consider this communication of a nature interessing to your nnme- 

 ous readers, I shall be happy to see it inserted in your very useful 

 fournal. 



If an open vessel containing water is placed over a fire, the water 

 will take up heat and will retain the same until it boils, after which 

 the water will throw off with the steam exactly the same quantity of 

 beat that it takes up from the fire. 



If the steam thus generated under atmospheric pressure is forced 

 into a worm contained in the water, so as to acquire a pressure of 

 about one twelfth of an atmosphere, it will be condensed therein at the 

 rate of about 3tb. of steara per hour for every superficial foot of re- 

 frigerating surface of the worm, and as by condensation all the latent 

 beat of the steara will be given up to the water, a corresponding eva- 

 poration thereof will be effected, so that providing there was no loss 

 of heat by radiation or other leakage, a liquid once brought into a 

 state of ebullition might be constantly kept in that state by its own 

 steam alone ; loss is hoivever inevitable, so that the liquid can be kept 

 boiling, only with the addition of a sufficient quantity of heat to com- 

 pensate for the loss by radiation, &c., and with the assistance of the 

 power requisite to compress the steara within the worm, this com- 

 pression can be effected either by means of a pump, or by a blast of 



high steam; the action of a pump is too well understood to need any 

 explanation, I will therefore merely describe the mechanical action of 

 a blast of high pressure steam empli'yed as a substitute for a pump. 



A blast of high pressure steam rushing through a tube of a greater 

 diameter than that of the blast itself, possesses the two valuable pro- 

 perties of producing a pressure at one end of the tube and a partial 

 vacuum at the other end thereof; when steam of about four atmo- 

 spheres is employed for the blast, and when the diameter of the tube 

 into which the blast rushes is about five times the diameter of the 

 blast itself, a pressure may be obtained at one end of the tube of about 

 S inches of mercury, and there will be found behind the blast a vacuum 

 nearly as powerful, — thus 



"^ 



CE 



tl 



The above sketch will give a correct idea of the blast apparatus. 



s, steam pipe when diameter = \. 



i, tube through which the blast rushes diameter = 5. 



a', vacuum gauge. 



a, pressure gauge. 

 With steam of about 4 atmospheres the gauge a will mark about S 

 inches of mercury, and the gauge a' something less. 



Now if the branch pipe b is connected to the steam chamber of an 

 evaporating pan in which the steam is produced under atmospheric 

 pressure only, the blast of high steam as above described, will absorb 

 from this steam chamber about four times its own volume (when re- 

 duced to about one-sixth of an atmosphere), and will compress this 

 mixture of high and low steam within the worm giving to the whole 

 a pressure of about one-sixth of an atmosphere. 



By the above application the water in the evaporating pan will be 

 evaporated by the steam produced therefrom, and the whole value of 

 the blast, minus that quantity required to compensate for leakage, will 

 be thrown off with the condensed steam through a valve placed at 

 the extremity of the worm, and may be utilized in a separate vessel to 

 heat the liquid to be evaporated previous to its being admitted to the 

 evaporating pan. 



The first experiments were made with a double acting pump which 

 drew the steam from the surface of the liquid as fast as it was gene- 

 rated, and forced it into the worm so as to determine an internal pres- 

 sure of about one twelfth of an atmosphere, the area of the piston was 

 TG square inches, and its greatest possible stroke 9| nches, but as the 

 pump was worked by hand the stroke was very irregular. 



The pump made 800 strokes in 35 minutes, each stroke being nearly 

 complete ; had each stroke been complete, and supposinof that there 

 was no loss by the valves, we should have obtained from the valve at 

 the end of the worm. 



9-875 X 2 X 7G X 800 



1728 



700 



: 700 cubic feet of steam, 



180U' 



; 0'39 cubit feet of water, 



or 0*39 X 62-5 = 24 16. avoirdupois of water. 



The quantity of water really discharged from the valve was 2016. 

 avoirdupois, which was necessarily the real quantity evaporated from 

 the pan. The fuel consumed did not exceed a quarter of a pound, we 

 consequently evaporated SU 16. of water with 1 16. of coal; this ex- 

 periment was repeated several times with the same result in an eva- 

 porating apparatus heated externally by fire, and containing a worm 

 for the action of the steam, the surface of which amounted to 21*5 

 square feet. 



An apparatus of this kind might be applied with great benefit at 

 sea to distill sea water, no machinery or steam boiler being required, 

 the apparatus being worked by hand, one man would obtain about 2-6t> 

 imperial gallons of distilled water every hour — the pump could be 

 worked in many different ways. 



