1841.J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



47 



tual in fitting and holding the grains together, so as to form a plastic 

 mass bv uniting the grains of sand which otherwise would not fit each 

 other. This system will apply to lime-mortar for all descriptions of 

 work, whether for l)uilding, plastering in the inside or outside of 

 houses, water cistenis, ground vaults, rough casting, &c. Szc. 



It may not be improper to mention that whenever there is any diffi- 

 culty in procuring proper sand for building, clay is an excellent substi- 

 tute ; and all that is necessary is, to make it into balls, and burn it, and 

 then pound it like brick-dust, or pozzolano earth. There is no doubt, 

 in addition to the superior scheme of making mortar in former ages, 

 that, when they used only the small stone, which we see in the ruins 

 of their buildings, they were in the practice of using temporary casings 

 of boarding which they could move from place to place as the build- 

 ing advanced, and which would enable them to grout or fill up vfith 

 their quick mortar all the interstices in the successive layers of stones. 

 And moreover, by having the boarding of their centering for arches 

 and conduits quite close, they were enabled to lay on, along with their 

 stone, almost an impenetrable coating of plaster. 



From the foregoing observations, it is hoped, it will be most clearly 

 seen that an easy mode of erecting substantial and durable building is 

 generally within our reach, and that the most inferior kind of stones 

 may be used, providing proper care is taken in the preparation of the 

 lime-mortar with which they are to be cemented together. 



John Gibb, 



Aberdeen, January 2, 1841. M. Inst. C. E. 



ON THE CONSTANCY OF CALORIFIC ABSORPTION, 



EXERCISED BY THE BLACK OF SMOKE ANT) BY METALS ; AND ON THE 

 EXISTEN'CE OF A DUTUSIVE POWER, WHICH BY ITS VARIATIONS 

 CHANGES THE VALUE OF THE ABSORBING POWER IN OTHER ATHER- 

 MANTIC BODIES. 



( Translated from the French q/'M. MELLONi,_/br the C. E. & A. Jour.J 



There were great difficulties to be overcome in the attempt to 

 prove that the black of smoke, subjected to the action of diflferent 

 kinds of radiating heat, always absorbs the same proportions of them. 

 The question would be immediately solved if we could_ successively 

 expose the blackened body to equal radiations, drawn from several 

 sources of caloric ; for a thermometer plunged in the interior of the 

 body would show by the greater or less elevation of temperature, 

 whether the quantities of heat absorbed vary or not with the quality 

 of the incident heat. When however we come to use the thermometer 

 or thermoscope in experiments on radiating heat, it becomes necessary, 

 as we shall hereafter see, to cover them with the black of smoke. On 

 the other hand, to compare two forces, whatever the effect which they 

 produce upon the measuring instrument must be estimated exactly in 

 proportion to their intrinsic energy. Thus we cannot compute the 

 relative intensity of rays of heat but by admitting the principle in 

 question : the experiment therefore of a thermometer plunged in the 

 interior of the body would be quite illusory. 



The first operation is to take a disc of wood, of which one face is 

 white and the other black ; this is fixed vertically upon a stock move- 

 able upon its axis, and having successively brought the two surfaces 

 by a half revolution of the disc in presence of the radiation of a lamp 

 concentrated by a glass lens, each time is collected with a very sensible 

 thermometer provided with a reflector, the secondary calorific radia- 

 tion projection by the side on which the direct rays fall, after this ra- 

 diation has traversed a plate of glass interposed between the disc and 

 the thermometer. In the case of the black face there is no sign of 

 heat; but things are different with regard to the white face, from 

 which is obtained a very intense indication of caloric. It is well 

 known that white bodies can never be heated more than black bodies 

 under the influ>ncc of any radiation whatever, and under the circum- 

 stances of the experiment the black face gives nothing; therefore the 

 great action of the white face does not arise from the absorbed heat, 

 but from a true dispersion, similar to the diffusion suffered by luminary 

 rays and the exterior of opaque bodies. To prove the variable diffu- 

 sive action which a white surface exercises on calorific rays from dif- 

 ferent sources, and the constant absorption of the black of smoke in all 

 kinds of heat, a very sensible thermometer is used with a reflector, 

 carefully sheltered from rays direct from the source and by it are mea- 

 sured the true secondary anterior and posterior radiations projected 

 from the surface of an immoveable disc subjected to a given radiation. 

 The same observations are repeated for several kinds of heat by em- 

 ploying two discs of thin cardboard, one painted black and the other 

 covered with a substance more or less white The first of these discs 



constantly exhibits the same relation between the rays vibrated by the 

 two faces, the second shows very different relations. Underneath is 

 shown the relative results of four species of rays arranged according 

 to the order of the temperature, of the sources from which they 

 emanate. 



Black disc 

 White disc 



-it JJi OJi 10 



13 T5 TJ tS 



2i> 10 10 10 



TT Ti? inr n 



In order to enable us to draw conclusions from tliese figures, it is for 

 the present to remark that the posterior face of each disc radiates in 

 consequence of the heat absorbed while the anterior face acts at the 

 same time by virtue of the radiations caused by absorption and diffu- 

 sion ; we therefore see 1st. That the black of smoke absorbs and dis- 

 perses all kinds of calorific rays with the same energy. 2nd. That the 

 diffusibility of caloric on the surface of the white disc increases with 

 the temperature of the source. 



As a detail of the other experiments would require too great a space, 

 it will be sufficient to sum up here the general results. 



1. The superficial layers of bodies cause to radiating heat a disper- 

 sion analogous to luminous dispersion. 



2. We possess sure means of distinguishing calorific diffusion from 

 the radiation derived from the proper heat of the body, notwithstaud- 

 iag both radiations are equally composed of elementary pencils radia- 

 ting in every direction around the centre of action. 



3. The black of smoke produces very little diffusion equal for all 

 kinds of radiations. 



4. That other substances, and especially white bodies are very dif- 

 ferent, as they strongly disperse rays from incandescence, and weakly 

 disperse those which derive their origin from sources of temperature. 



5. This special characteristic is enough to show that we must not 

 attribute the phenomenon of calorific difl'usion to every regular or irre- 

 gular reflection whatever ; for this would take place with the same 

 energy for all kinds of heat. 



6. The dispersive action of metals is generally speaking more ij!« 

 tense than that of white bodies ; it especially differs by its invariability, 

 and on this point resembles the feeble diffusion observed in the black 

 of smoke. 



7. By comparison between the phenomena of calorific diffusion and 

 those of luminous diffusion, it appears 1st. That the black of smoke is 

 a true black matter, both as regards radiating light and heat. 2nd, 

 That white bodies act with regard to radiating heat as coloured sub« 

 stances with regard to light. Srd. That metals act upon calorific ra- 

 diations as white bodies do upon luminous radiations. 



S. The diffusion sends back a part of the incident rays proportion- 

 ate to its intrinsic energy, and thus diminishes the calorific absorption 

 of the whole portion of heat dispersed by the action of the surface. 



THE PNEUMATIC MARINE PRESERVER. 



Sir — In viewing the many interesting and scientific exhibitions at 

 the Polytechnic Institution in Regent Street, my attention was more 

 particularly arrested by the model of a ship fitted up with a new in- 

 vention, called the Pneumatic Marine Preserver ; indeed, I was as- 

 tonished to see the little vessel, though full of water and cargo, still 

 keep afloat, and to a casual observer's eye, without the least aid, as 

 the apparatus occupies so little room, and is so placed out of the way, 

 that ninety-nine out of a hundred would not observe the reason of its 

 buoyancy. While carefully examining the craft, a person who shows 

 it to the public, suddenly exhausted the air, and she gradually sunk 

 completely out of sight ; but to my surprise, by a few strokes from 

 the condensing air-pump, she immediately rose to the surface of the 

 water, and again floated about. 



Being an old sailor, I thought it a duty I owe to my fellow creatures 

 to make it known to the public through your valuable Journal : and I 

 would particularly advise captains to have their boats fitted up vrith 

 the Patent Pneumatic Marine Preserver, as, in case of danger, they 

 then become perfect life-boats, far superior to any yet invented for 

 room, lightness, and buoyancy. If ever there was an invention of 

 incalculable service to sea-faring men this is the one, and deserving of 

 their utmost attention if they value life or property. 



I remain 

 London, Your obedient servant, 



January, 1841. An Old Sailor. 



