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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL 



[Septembeb, 



He expeL'tfd, from the well-known sagacity of the Scottish 

 lieople, that when truly embarked in this mode of operation the 

 ^oeatei^t possible use would be made of it : and he would not be 

 Kurprised to see heat let out, like mill-power, fur burning bricks 

 and other similar purposes. He felt, however, anxious that the 

 application sliould be made under the superintendence of com- 

 petent parties, as he had known several instances where the plan 

 hid been abandoned from difficulties that might easily have been 

 surmounted under proper directions. He was quite aware that, 

 by the plan lie had pursued, the utmost heat was not extracted 

 IVom tlie gases; and tliat, by different means, a temperature might 

 he obtained capable of performing all the operations of the forge; 

 and if it be true tliat tlie solid carbon of the furnace in its escape, 

 as carlionic oxide, would unite with another dose of oxygen for 

 saturation, tliere could be little doubt that, with properly consti- 

 tuted gas furnaces, there was enough at present passing off to 

 convert the pig iron into bar iron. He hoped some of the iron- 

 masters of Scotland would follow up this hint effectually with 

 regard to the remaining processes required for making ma'llealde 

 iron. He observed that the saving at the Dundyvan Iron-works 

 was stated to be about 1] ton of coal for each ton of iron produced. 

 Supposing, therefore, 600,000 tons of iron to be the produce of 

 Scotland, and supposing the value of the coal used to be 3.v. a ton, 

 the saving that would tlius be effected on tlie make of Scotland 

 would amount to 112,500/. a-year; to which might be added 

 20,000/. a-year of saving in wages and repairs, which would make 

 a total saving of 132,500/., or about 4s. 5(1. a ton on the produce of 

 Scotland, which on the present price of 14.?. per ton, was about 

 10 per cent, on the value. If the gaseous escape could be ex- 

 tended to the uses of the forge, a farther saving of tliree tons of 

 i:oal would be effected — tlius making, at least, a saving of 20i. a 

 ton on all the iron manufactured into bars, sheets, and rails. 



ELECTKICirV AXD HEAT AS MOVIXG POWERS. 



On the Appiication of Electricity and Heat as 3[oving Powers. By 

 Mr. Petrie. 



From the dynamic equivalent of electricity, we can infer an 

 important fact that one-horse power is the theoretic or absolute 

 dynamic force possessed by a current of electricity derived from 

 the consnmpt of 1-56 lb of zinc per hour in a Daniells' battery. 

 But the best electro-magnetic engine that we can hope to see con- 

 structed cannot be expected to give more than half or a fourth of 

 this power; in any case we see here the limit of power wliich no 

 jietfection of apparatus can make it exceed. The peculiar mode 

 in which the electric current produces dynamic effects has led to 

 much miscalculation respecting the power obtainable from it. In 

 .my sort of electric engine the material to which the neighbouring 

 current gives motion, whether it be another moveable current, or 

 what is more usual, a magnetic body, is impelled in one direction 

 with a constant force, and this force, whether it be attraction, 

 repulsion, or deflection, is, like the powers of gravity, sensibly 

 constant at all velocities, however fast the body recedes'bcfore the 

 action of the force, provided only the same quantity (per minute) 

 of electric current be maintained! This is quite different from the 

 action of steam power, in which the faster the piston moves the 

 greater is the volume of steam per minute that must be supplied 

 to move it, or else the less will be the power with which it moves. 

 — This fact, then, that the force with which an electric current of 

 a given quantity moves the machine, is the same at any velocity of 

 motion, bears no analogy to the case of steam, but would indicate 

 that the dynamic result obtainable from a given electric current 

 might be infinitely great; and so it would be, were it not that tlie 

 part moved always tends to induce a current in the wire in the 

 reversed direction, and this inducing influence, which increases 

 with the velocity of motion, conflicts with the original current and 

 reduces its quantity, and consequently reduces the power of the 

 motion, as well as the consumpt of materials in the battery. Some 

 have imagined that possible alterations in the position of the parts 

 of the machine, or in its mode of action, would avoid the evil or 

 even might make the induced current to flow with the primary 

 current instead of against it; the impossibility of this, though not 

 readily ]iroved in detail, can be at once proved by reference to 

 general principles. It would, if true, be a creation of dynamic 

 force — the evolving an unlimited force from a limited source. 

 The tendency to an opposing induced current in the primary wire 

 must, therefore, be involved in tlie very iirinciple of the sj-stem; 

 so that no ingenuity can ever get rid of the retarding influence of 

 tlie induced action; and the only way to overcome its power, so as 



to maintain the primary current from falling below a given rate or 

 quantity wlien tlie machine is allowed to attain rapid motion is to 

 increase the electro-motive power of the battery, the intensity 

 (not the quantity) of the current, so that it should be less aifected 

 by the opposing induction. 



The practical importance of these not altogether unknown 

 truths, may justify the above somewhat particular notice of them. 

 For want of a clearer apprehension of them, inventors have mis- 

 apprehended the direction in which improvements were to be made 

 and much ingenuity and means have been wasted. 



Some of the best electro-magnetic engines of other inventors 

 that have been properly tested by the author and others, on a prac- 

 tically useful scale, have only given a power at the rate of .50 to 60 

 pounds of zinc per horse-power per hour. The sniallness of this 

 power in comparison with the aljsolute value of the current (1-56 

 pound of zinc per horse-power jier hour) should not occasion sur- 

 prise if we consider the present case of steam after many years of 

 improvement. 



According to the determinations of Youll and of Rankine on 

 heat, one pound of water raised one degree of temperature, is 

 equivalent to 7001b. weight raised one foot. The author then 

 proceeded to show that the bust Cornish engines only yield ^th 

 of the power that the combustion of the carbon actually represents, 

 and many locomotives only yrnith part ; — showing what great 

 rewards may yet await the exercise of inventive genius in this 

 department, and that we need not wonder that we have, as yet, 

 only obtained g-'^nd part of the power possessed by electricity. 

 But it is to be remembered that there is a far greater likelihood of 

 obtaining a larger proportion of the real power from electricity 

 than from heat, owing to the character of the two agents. 



Mr. Petrie then proceeded to explain the reasons why so little 

 of the power of heat could be obtained in a useful form, even in 

 the best steam-engines, and what vvere the difficulties for invention 

 first to overcome in order to a better result. 



In the case of electricity, however, there is no analogous diffi- 

 culty ; but we have instead, the difficulty and expense of developing 

 current electricity by the chemical actions now requisite. If 

 carbon could be burnt or oxidised by the air, directly or indirectly, 

 so as to produce electricity instead of heat, one pound of it would 

 go as far as 9^- pounds of zinc (in a Daniells' battery) chiefly 

 because there are as many atoms in one pound of carbon as there 

 are in 5| pounds of zinc, and partly because the affinity (for 

 oxygen) of each atom of (incandescent) carbon is greater than 

 that of an atom of cold zinc, minus the aiiinity of the hydrogen for 

 the oxygen in the water of the battery. Apart, however, from 

 such prospects of improved means of obtaining electricity, its 

 favourable feature, on the other hand, in comparison with heat, is, 

 the reasonable expectation that we may obtain from electricity a 

 considerable portion of the power which Mr. Petrie has determined 

 as being the dynamic equivalent of the electric current. 



REVOLVI.NG LIGHTHOUSE LIGHTS. 



On the Limits to the Velocity of Revolving Lighthouse Apparatus 

 caused bi/ the time required for the production nf Luminous Impressions 

 on the Eye. By A\'ii.liam Swan, F.R.S.E. 



The object of this communication is to ascertain the greatest 

 velocity that can be given to a revolving lighthouse apparatus, 

 without impairing the brightness of the light. It is well known 

 that at a given distance the apparent brightness of a revolving 

 liglit e.vcee<ls that of a fixed one, supposing the intensity of the 

 source of illumination to be the same in both cases; and this effect 

 is due to the fact that the revolving apparatus collects all the light 

 into beams of nearly parallel rays, which illuminate only a small 

 portion of the horizon at any instant, while the fixed apparatus 

 scatters its rays over every point in the horizon. The question 

 might occur, is it possible to continue the superior intensity of the 

 revolving with the constancy of a fixed light by increasing the 

 velocity with which the apparatus revolves so as to cause its flashes 

 to reach the eye in rapid succession? The attempt to combine in 

 this manner the advantages of the two systems of lights was 

 actually made by the late Captain Basil Hall, who, founding on 

 the well known jihenomenon of the persistence of impressions on 

 the retina, conceived the ingenious idea of causing a revolving 

 light to rotate so rapidly as to produce a continuous impression on 

 the eye. 



The ])ractical efficiency of this arrangement was tested by Mr. 

 Alan Stevenson; and the result of his experiments is described in 



