26 



NATURE 



{Nov. 14, 1878 



the usual engineering practice to so distribute the stresses 

 that no joint tends to open under the most unfavourable 

 conditions, though this condition is doubtless frequently 

 neglected in flimsy structures. In order that this con- 

 dition should be fulfilled, the resultant of the pressure 

 on the base must not deviate from the centre of gravity 

 of the base by a quantity greater than x' given in the 



equation x' = jl^-, where / is the moment of inertia of 



the base about the neutral axis or line through its centre 



of gravity perpendicular to the direction of the deviation 



of the resultant, vS" = the area of the base, and X = the 



greatest distance of a point in the base from the neutral 



axis on the side of the greatest pressure. In the case of 



diameter of base 

 a circular base x' = s = •625 feet m 



the present instance. The wind pressure corresponding 



. , . J . . 196 X '625 X 2240 



!to this deviation = = 21 "22 lbs. per 



square foot. When the wind-pressure exceeds this 

 amount there is still the tensile strength of the cement 

 with which the stone is bedded to resist the tendency of 

 the joint to open on the windward side. While the in- 

 troduction of a layer of cement under the stone doubtless 

 adds to its steadiness under a wind-pressure of 30 or 

 50 lbs. to the square foot, it would add a very serious 

 element of danger should the pressure ever approach 

 that recorded at the Bidston Observator)', as the cement 

 on the lee side would probably then be subjected to a 

 crushing stress in excess of its strength, and by giving 

 way would cause the column to heel over to some extent ; 

 in fact, if there was any probability of that wind-pressure 

 being reached, it would have been safer to hare omitted 

 the cement and trusted for the ultimate stability to the 

 Szr greater resistance to crushing of the granite. It 

 would be impossible, without making assumptions un- 

 •founded on experiment, to estimate with any accuracy 

 the value of the additional stability given by the cement 

 in the case of moderate wind-pressures. We have, how- 

 ever, calculated the conditions of equilibrium, neglecting 

 the tensile strength of the cement, as well as the bending 

 of the stone. 



On this assumption, we find that a wind pressure of 

 jolbs, per square foot would cause the joint to open on 

 the windward side as far into the base as the centre ; the 

 column would thus be standing only on the leeward half 

 of its base, but the stability would not be endangered by 

 this as the maximum pressure on the base at its outer 

 edge would only amount to 40 tons per square foot, w^hich 

 is less than the crushing strength even of the cement. 

 The line of the resultant pressure on the base would be 

 -at a distance of i "472 feet from the centre, if the bending 

 of the column is disregarded. To take into consideration 

 the flexure of the column would involve too long calcula- 

 tions for our present purpose, even if the modulus of elas- 

 ticity of granite had been determined with sufficient 

 accuracy to make the results of any value, but this we 

 believe has not yet been done. The conclusions we arrive 

 at are as follows : — As long as the foundations remain 

 secure, the obelisk may be frequently subjected to a wind 

 pressure of 21 lbs. per square foot without the slightest 

 tendency to accident ; if subjected at long intervals to a 

 pressure of 40 or 50 lbs. to the square foot, it would 



probably stand for an indefinitely long period until the 

 fatigue of the cement under variations of stress or its 

 natural decay, if that ever takes place, causes its rupture, 

 but under a pressure of this intensity it must be borne in 

 mind that considerable oscillation would take place, and 

 that if the period of the gusts nearly agreed with the time 

 of vibration of the stone it might be overturned ; while if 

 a pressure of 80 lbs. per square foot is reached it is very 

 questionable if the survivors among the inhabitants of the 

 neighbourhood will find it in situ when they hare time to 

 go and look for it. 



DRAPER'S SCIENTIFIC MEMOIRS 



Scientific Memoirs : being Experimental Contributions to 

 a Knowledge oj Radiant Energy. By John William 

 Draper, M.D., LL.D, (London : Sampson, Low, and 

 Co. New York: Harper Brothers, 1878.) 



THE scientific world is . to be congratulated on the 

 accession to its literature of these memoirs con- 

 stituting as they do a distinct historical sketch of thi 

 works of a physicist who is at once an ardent experi- 

 mentalist and a careful theorist. As he remarks in hi 

 preface, many of his results of experimental investigatioi 

 on scientific topics have been largely disseminated ii 

 European languages, and many of the conclusions they 

 have presented have been admitted into the accepted 

 body of scientific knowledge. The papers in which these 

 results were published have, however, appeared from 

 time to time in various American and English periodicals, 

 but we now have them collected in a form in which they 

 are accessible and convenient for reference. 



The four opening memoirs seemingly occupy their 

 position in the volume for the purpose of calling the 

 attention of the reader to the fact that a large portion of 

 the subject that Kirchhoff treated mathematically in a 

 paper which appeared in Poggendorff's Annalen in i860, 

 and which at the time was considered the foundation of 

 spectrum analysis, had already been experimentally proved 

 and published by our author some thirteen years before- 

 The theorist apparently ignored the work of the experi- 

 mentalist, and the claim of the one to priority in regard 

 to the enunciation of certain fundamental principles of 

 spectrum analysis is now on the best of evidence disputed 

 by the other. The titles of these first four memoirs and 

 their dates of original publication will give an idea of 

 the indictment framed against Kirchhoff which appears 

 in a note appended to the last of them. They are — 



I. Examination of the radiations of red-hot bodies. The 

 production of light by heat, published in 1 847. 



II. Spectrum analysis of flames. Production of light 

 by chemical action, published in 1848. 



III. On invisible fixed lines in the sun's spectrum 

 detected by photography, published in 1843. 



IV. On the nature of flame, and on the condition 

 the sun's surface, published in 1858. 



Controversy regarding priority of discovery is alwa 

 distasteful, and the indictment against Kirchhoff is a heavy 

 one, but the offence might have been charged also against 

 those scientific writers who, careless of history, have been 

 accomplices in doing Draper an injustice. But turning to 

 the more agreeable side of the subject of these memoirs 

 we find that Draper fixed the temperature at which solid 



i 



