May 2, 1878] 



NATURE 



19 



POZZOLANA MORTAR AND PINE TIMBER 



THE following letter has been sent us for publication 

 by Prof. Tyndall :— 



Villa Guastalla, Via Palestro, Rome, April 14 



Sir, — A very curious and unexpected circumstance has 

 occurred in Rome, which, as it depends on chemical 

 action, may have some interest for you. 



Prior to 1870, when Rome became an integral part of 

 the kingdom of Italy, the beams used in the construction 

 of houses were of chestnut wood. After that date a vast 

 amount of building was undertaken and now a whole 

 quarter of the city stands on ground formerly occupied 

 by vineyards and gardens. 



k In lieu of chestnut, pine was largely used, having been 

 brought via Venice from the Dolomite Alps. The latter 

 was preferable, as being procurable of larger scantling, 

 of greater length and at a less cost. 



After a few years, the roofs and floors in which the 

 pine had been used were found to be failing. A beam 

 used in a flat roof or in flooring, where it was imbedded 

 in a wall was found to be rotten ; while the body of the 

 beam was perfectly sound. A very considerable sum of 

 money was thus lost, as many of the roofs and floors of 

 the new houses on the Esquiline had to be renewed. 



But what was the cause of this sudden perishing of the 

 ends of the pine beams, such as had been known to 

 last centuries in Venice ? The answer to this question 

 remained a puzzle for a long time ; until on taking down 

 the scaffolding of the Ministry of Finance lately com- 

 pleted, a complete answer was found. 



One of its scaffold poles had been imbedded for, say, 

 four feet in the ground ; about its foot was a heap of 

 the debris of Pozzolana mortar, say, six feet high. That 

 part which had been underground was perfectly sound ; 

 that which had . been surrounded by mortar was utterly 

 rotten ; and finally, the remainder of the pole above the 

 ground was perfectly sound. Hence, it was clear that 

 the mortar was to blame. But in what respect did this 

 mortar difter from that used at Venice in which pine 

 wood beams lay embedded for centuries with impunity ? 

 The sole difference was in the use of pozzolana — a vol- 

 canic earth — instead of sand, and as this substance had 

 been used for mortar in Rome and Naples for ages in 

 contact with chestnut beams with impunity, the only 

 logical conclusion is that pozzolana and pine wood have 

 some chemical affinity which causes some of their in- 

 gredients to combine, to the destruction of the latter. 



Inclosed are a few grains of pozzolana, such as is used 

 for mortar in Rome. 



Yours faithfully, 

 Henry H. Maxwell, Lieut.-General R.A. 



Dr. Tyndall 



STANFORD'S STEREOGRAPHICAL MAP OF 

 THE BRITISH ISLES 



ly/TR. STANFORD has recently issued a map which 

 ^^^ marks a distinct advance in British cartography, 

 and one which gives us ground for hoping that 

 some day we may be able to equal in this country the 

 work of the geographical establishments of Germany. 

 The map in question represents, in the first place, the 

 United Kingdom, with its hills and mountains standing 

 solidly out from the ground, as if a perfect relief 

 model of the country lit up from the North had been 

 photographed. The plains and valleys are also clearly 

 shown ; on ordinary maps these cannot be distinguished, 

 and yet they are as important features as the hills them- 

 selves. 



Great 'care has been taken to embody all the usual 

 information without in any way detracting from the 

 beauty of the map. Thus the railways are shown, and 

 cities and towns, so as not to interfere with the physical 



features, as well as the hills and plains, vales and rivers, 

 are named in a clear yet delicate type. 



As an example of the information conveyed, we can 

 mark in the map how the giround rises gradually in going 

 west from London all the way to the summit ridge of the 

 Chiltern Hills, and then falls suddenly to the Vale of 

 Aylesbury and the Vale of the White Horse ; the ground 

 again rising gradually to the summit of the Cotswold 

 Hills, and then falling suddenly to the valley of the 

 Severn ; how the headwaters of the Thames all lie on 

 the top of the second ridge, while the first ridge is the 

 boundary between the Upper and Lower Thames Valley, 

 presenting only one vulnerable point, between Walsing- 

 ham and Reading, through which the river can make its 

 way. 



Mr. Stanford claims that the map is at the same 

 time artistic and scientifically accurate ; and from the 

 examination we have made we believe both claims can be 

 well made out. 



OUR ASTRONOMICAL COLUMN 



Transits of Mercury. — After the transit of Mercury 

 across the sun's disc on Monday next. May 6, which will 

 be visible in this country through about half its duration, 

 there remains only one transit of this planet at the de- 

 scending node in the present century ; it will take place 

 on May 10,1891, with the following elements according 

 to Leverrier's tables of sun and planet : — 

 G.M.T. of conjunction in R.A. 1891, May 9, at I5h. 55m. 40s. 



Whence the first external geocentric contact occurs at 

 iih. 53m. 19s. at 65° from the sun's north point towards 

 the west, for the inverted image, and the last external 

 contact at i6h. 52m. i8s. at 12° from the north point 

 towards the east. At Greenwich the external contact at 

 ingress takes place at 4h. 50m. 26s. A.M. on May 10, and 

 the sun's centre is in the horizon at 4h. i8'5m., so that 

 Mercury will be only half an hour upon his disc, after 

 observation is possible here. And while the egress of 

 the planet from the solar disc is alone visible in these 

 islands in the transit of 1891, in that of November 10, 

 1894, at the opposite node — the last phenomenon of the 

 kind in the nineteenth century — the ingress only can be 

 witnessed here, under faroiu-able atmospheric circum- 

 stances, not to be insured at this season ; the first ex- 

 ternal contact at Greemrich taking place at 3h. 55m. P.M. 

 and the sun setting at 4h. i8m. 



At the sitting of the Paris Academy of Sciences on 

 April 22, a letter from M. Andrd was read, stating that 

 the expedition sent by the Academy and the French 

 IMinister of Public Instruction, to Ogden in the Utah 

 territory for the Observation of the Transit of Mercury 

 in the present month, had arrived safely at its desti- 

 nation. After experiencing very liberal treatment from 

 the French Trans-Atlantic Company, the instruments 

 were admitted without payment of duty at New York, 

 and the observers received free passes on the lines of 

 railway converging in Utah, both for the outward and 

 homeward journeys. The Government of W^ashington 

 placed at their disposal the nearly-finished observatory at 

 Ogden, at the same time undertaking to provide gratuit- 

 ously all necessary appliances for the observations. A 

 telegraphic wire from Washington to Utah was available 



