1841.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



317 



the mean difterence between it and tlie mean cylinder exhaustion can- 

 noi ^ossiV)/!/ have amomited to "2.41 tti., tlie external barometer standing 

 at 30 inclies, as that would indicate a perfect vacuum, which is ob- 

 viously impossible.* 



In Mr. Pilbrow's engine the ordinary condenser and air pump are 

 replaced bv a double-acting air-pump of the same size as the steam 

 cylinder, called the condensing cylinder, in the interior cf which the 

 condensation is etiected by injection alternately above and below the 

 piston, which is of course solid, like the steam piston. The two cylin- 

 ders are connected at top and bottom by passages, with valves to open 

 and close the communication alternately. The action will be as fol- 

 lows: while the steam piston is ascending, the air-pump piston is 

 descending, and the two cylinders communicating at top, the steam 

 ■which performed the previous down stroke will flow into the condens- 

 ing cylinder, and be condensed by the jet, by which, as we know from 

 the experience of ordinary condensing engines, the vacumn above the 

 air-pump piston will be maintained at nearly its maximum, while the 

 exhaustion of the cylinder will be nearly the same as in ordinary con- 

 densing engines. Mr. Boyman however supposes that, "during its 

 condensation, the uncondensed steam will keep giving to the condenser 

 piston, until completely annihilated, just as much power as it offers 

 resistance to the effective action of the steam piston." The exhaustion 

 on the under side of the condenser piston will be the maximum through- 

 out the stroke, so that the resistance to the motion of the steam piston 

 (exclusive of friction and the resistance to the discharge of the con- 

 densement in the latter part of the stroke) will be equal to the mean 

 pressure of the used steam remaining in the steam cylinder minus the 

 difference between the maximum and mean exhaustion in the condenser, 

 and this difterence, which is quite insignificant, is, after deducting the 

 surplus power required to work his large air-pump, the true gain of 

 power obtained by Mr. Pilbrow's contrivance, and we think it probable 

 that after the deduction the gain w ill be found to be negative, or a 

 loss. 



CARBONIC ACID GAS VERSUS STEAM. 

 fFrom " Buckingham's Amerha.J 



Towards the close of our stay in Pliiladelphia, I had an opportunity of 

 attending one of the chemical classes of mv friend Dr. Mitchell, and witness- 

 ing there a most interesting experiment for the rendering carbonic acid gas 

 solid, and for producing by it a degree of cold, extending to 102 degrees be- 

 low zero, on the scale of Fahrenheit's thermometer. The materials, first 

 confined in a strong iron receiver, were, super-carbonate of soda and sul- 

 phuric acid, in separate divisions; the whole was then powerfully shaken, so 

 as to be well mixed or incorporated, and this operation continually evolved 

 the gas, tiU the whole vessel was filled with it in a higldy condensed state. 



An instrument not unlike a common tinder-box, as it is used in England, 

 but about twice the size, and with a small tube of inlet passing through its 

 sides, was then fixed by this tube to a pipe from the receiver. The inside 

 of this box was so constructed as to make the gas injected into it fly round 

 in a series of constantly contracting circles, which was effected by projecting 

 pieces of tin at different angles, fastened around the sides of the interior. 

 The gas being then let out by a valve, entered tbis box from the receiver, 

 making as loud a hissing noise as the escape of steam by the safety-valve of 

 a large boiler, and in about three or four seconds the emission of the gas was 

 stopped. 



The box was then taken off from the receiver and its cover opened, when 

 it was found to he filled with a milk-white substance, in appearance like 

 snow, but in consistence like a liighly-wrought froth, approaching to a light 

 paste. It was surrounded with a thin blue vapour like smoke, and was so 

 intensely cold, that the sensation of touch to the fingers was like that of 

 burning; and the feehng was more like that of heat than cold. The shghtest 

 particles of it dropped on the back of the hand, and suffered to remain there, 

 occasioned a blistering of the skin, just like a scald; and some of the stu- 

 dents of the class who attempted to hold it in their fingers, were obUged to 

 let it drop as if it were red-hot iron. 



Some liquid mercury, or quicksilver, was then dropped into a mass of this 

 " carbonic acid snow," as it was called, mixed with ether, upon which it 

 instantly froze, and being taken out in a solid mass, it was found to be mal- 

 leable into thin sheets under the hammer, and capable of being cut up like 

 lead, with a knife or large scissors. As it became less cold it grew more 

 brittle, and then, when pressed strongly by the thumb or finger against a 

 sohd substance, it was found to burst under the pressure, with a report or 

 explosion like the percussion powder. 



.\ small piece of this carbonic acid snow was placed on the surface of 

 water, where it ran round by an apparently spontaneous motion, and gave 



* It may be as well to observe here that the dulerence between the ex- 

 haustion in the cylinder, and in the condenser is independent of the mode of 

 condensation: and that consequently, if by any improved process the vacuum 

 in the condenser be' increased, the cylinder exhaustion must be so too. 



out a thin blue vapour like smoke. Another piece was placed under the 

 water, and kept beneath it, when it emitted gas in an immense stream of 

 air-lmbbles, lushing from the bottom to the top; thus returning, in short, 

 from lU solid to its original gaseous condition. Some of the snow was then 

 mingled with the well-known " freezing mixture," and bv stirring these both 

 together, ^ degree of intense cold was produced, extending to 102 degrees 

 below zero, and there remaining for a period of ten or fifteen minutes ; 

 tliough the weather was extremely hot, the thermometer standing at 94 

 degrees in the shade, in the coolest parts of Philadelphia, and being at least 

 90 degrees in the lecture-room itself. 



The practical application of this discovery to the propelling of engines in 

 lien of steam, was then exhibited to us. A model of an engine of the ordi- 

 nary kind now in use for mines, manufactories, and steam-ships, was placed 

 on the table before the lecturer. A metal tube was then screwed on to the 

 pipe and valve of the receiver, in which the condensed carbonic acid gas 

 was contained, and the other end of the tube through which the gas was to 

 escape, when let into it from the receiver, was applied to the wheel of the 

 model engine; the gas was then let out, and the rushing torrent of it was 

 such as that it propelled the engine wheel with a velocity which rendered its 

 revolutions invisible, from their speed, making the wheel appear stationarv, 

 though in a trembling or vibratory condition, and rendering all perceptioH 

 of the parts of the wheel quite impossible till the gaseous stream which gave 

 the impetus was withdrawn. 



Dr. Mitchell expressed his belief that this power might he made to super- 

 sede entirely the use of steam and fuel in navigation, and thus overcome the 

 greatest difficulty wliich has yet impeded long voyages; he thought it might 

 effect the same salutary change in manufactories wliere engines are used, so 

 as to remove the greatest nuisance, perhaps, of all manufacturing towns, the 

 inunense quantities of smoke which darken the atmosphere, and destroy the 

 cleanliness of places, persons, raiment, and dwellings. He founded his be- 

 lief on the expansive power of this gas when brought into a highly condensed 

 state, such as we saw it, and the practicabdity of bringing this power to act 

 upon engines of any size by land or by sea. For the latter purpose he sug. 

 gests the use of iron tanks, made with the requisite degree of strength, to 

 act as receivers; these being fitted to a ship's bottom, along the keelson and 

 the inner floor of the hold, as the iron water-tanks of ships of war are at 

 present, it may be placed on board vessels intended to be propelled by engines, 

 in such quantities as the length of the voyage may require ; communications 

 from these tanks, by tubes of adequate size and strength, would then have to 

 be made to the engines, and placed under the complete control of the engi- 

 neer, as the steam-pnwer is at present. The expansive power of the con- 

 densed gas, and its pressure outward, or tendency to escape, being the same 

 in its nature with steam, but greater in degree, the application and direction 

 of tbis power would effect all that steam now does, and thus supersede the 

 use of fuel, with its inconveniences and accidents, entirely. 



In reference to the expense. Dr. Mitchell had made such calculations as to 

 satisfy him that it would be cheaper than tlie present materials of steam 

 navigation. The Great Western steamer, in coming from London to New 

 York, actually consumed GOO tons of coal, which, at the lowest possible 

 estimate, could not cost less than f 1000 sterhng, or 5000 dollars. But as 

 it was necessaiy to provide for a longer voyage than that actually performed, 

 in case of accident or delay, no less a quantity than 800 tons we're taken on 

 board, and consequently 800 tons of space were wholly lost, or rendered 

 unproductive, by its appropriation to fuel. The expense of the requisite 

 quantity of gas for such a voyage, including all the fittings, would not, he 

 thought, exceed that of the coals and requisite machinery; and the saving 

 of the space, for freight, would be a source of considerabie profit ; while the 

 avoidance of the beat and smoke, inseparable from fuel and steam, the ab- 

 sence of boilers and chimneys, and the safety from accidents of bursting and 

 taking fire, would be all such high recommendations to passengers, that none 

 would venture to embark in steam-ships while those propelled by carbonic 

 acid gas were available. 



PROCEEDINGS OF SCIENTIFIC SOCIETIES. 



INSTITUTIOX OF CIVIL ENGINEERS. 



March 16. — The President in the Chair. 



" Description of tvo Wrought-Iron Roofs over the buildings at Mr. Thomas 

 Cubiti's Works, Thames Bank." By Mr. Adams. 



This communication describes in detail the constniction, and gives the 

 dimensions of the several parts of two fire-proof roofs of 29 feet si)an, one of 

 which bears, in addition to the covering, a ceiling of tde arches upon iron 

 girders, the weight of which is equal to 5 tons 4 cwt. upon each truss. 



The paper is accompanied by two drawings of the roofs. 



" Description of a Double Telescope Theodolite." Arranged by Nathaniel 

 Beardmore, Grad. Inst. C.E. 



The improvement in this theodolite consists in its having a second telescope 

 fixed over the ordinary one, in a reverse position, so that the line of collima- 

 tion of the two telescopes when properly adjusted shoidd be the same. The 

 principal advantage gained is, that a straight line may be carried out with 

 perfect accuracy, without the tedious and uncertain process of adding 180 



I 



