5S 



HA RD WICKE' S SCIENCE- G OSS I P. 



explained by the thickness and tenacity of its shell, 

 which would resist until the elastic tension of the 

 gases within become formidable. 



Expansion and Contraction of Buildings. — 

 The use of iron and other metals in building con- 

 struction demands scientific consideration and skilful 

 arrangements, to meet the effects of varying tempera- 

 lure and consequent variations of expansion. Thus 

 tbe central arch of Southwark Bridge rises about 

 one inch in the summer, lifting with it the footpath, 

 roadway and all upon them, and is constructed to 

 allow this to occur without undue strain. The ends 

 of the tube of the Britannia Bridge across the Menai 

 Straits rest on rollers, to permit the advance and 

 recession due to heat and cooling, the length of the 

 bridge varying as much as three inches in the course 

 of twenty-four hours. When the sun shines on one 

 side of the tube, that side becomes longer than the 

 side in shade, and the whole structure curves ac- 

 cordingly. The great variation of temperature 

 between the extremes of summer and winter in 

 America causes the Brooklyn Bridge to vary in 

 length to the extent of several feet. Its total length 

 is 3,540 feet, and the temperature ranges from — 15 

 to +115 Fahr., a difference of 130 degrees. As 

 iron expands g ^ s of its length between the freezing 

 and boiling points of water (180 Fahr.) the amount 

 is easily calculated. The Washington Monument 

 leans to the east in the morning, and to the west in 

 the afternoon. A plummet line suspended in the 

 interior of the dome of the capitol of Washington 

 swings 4! inches from the perpendicular on each side, 

 or 8£ inches altogether. 



The Setting of Cement. — Among the abstracts 

 of foreign scientific papers published by the Chemical 

 Society, I find two on this subject, one by E. Michel 

 and one by L. C. Levoir, but neither of them contain 

 anything remarkably new. Any bricklayer's labourer 

 could tell us that " the setting of cement is dependent, 

 firstly, on the addition of water, which should be as 

 limited and uniform as possible ; and secondly, on 

 the density and intimate mixture of the materials." 

 In telling it Paddy's English might be somewhat 

 different, probably better, as it would not leave us 

 in doubt whether the mixture or the water should be 

 "as uniform as possible." The "thirdly," viz. the 

 conversion of the lime into carbonate, may not be 

 understood by him unless an ex-pupil of a Board 

 School. The most important chemical action which 

 occurs in the setting of cements is the union of the 

 lime with silica, whereby a stony compound, the 

 silicate of lime, is formed. The simplest of cements 

 is common mortar, i.e. lime mixed with silica in the 

 form of sand, the finer the sand and the more caustic 

 the lime (i.e. the more free from carbonate) the better. 

 The jerry mortar now used so villainously in London 

 suburban building is worthless, because dust-hole 



siftings or ballast (burnt clay) is used instead of sand 

 by swindlers who still remain unhanged. M. Levoir 

 fully recognises the importance of the formation of the 

 silicate in reference to Portland cement. The obscure 

 part of the subject is the difference between hydraulic 

 and ordinary cements ; why some cements may be 

 immersed in water immediately after they are mixed 

 and harden nevertheless, while others would go to 

 slop under the like conditions. I refer, of course, to 

 cements having lime as their basis, not to the glue- 

 like compounds sometimes described as cements. 



Meteorite in Coal. — If the fall of meteoric 

 stones is an old-established proceeding, there must 

 be specimens imbedded in the stratified rocks of all 

 ages. This may be the case although none have 

 yet been found, for the simple reason, that the total 

 area of any given formation which human beings 

 have explored bears so small a proportion to the 

 total area of the globe. The non-finding of them in 

 rocks of any particular date no more proves their 

 non-falling at that period than the non-finding of 

 specimens on the present surface of the county of 

 Middlesex proves their non-falling in the present 

 geological epoch. As the best-explored regions of the 

 geological past are those containing coal, it is there 

 that we may expect the first find, and in accordance 

 with this natural probability a fossil meteorite has been 

 found in a block of Tertiary coal. It was described 

 by Dr. Gurlt, at Bonn. The block of coal from which 

 it was taken was about to be used in a manufactory 

 in Lower Austria. It weighs 785 grammes (about 

 1 fibs.) its specific gravity is 775 ; it is as hard as 

 steel ; is a mass of iron alloyed, as meteoric iron 

 usually is, with nickel, and combined with some 

 carbon. 



The Solar Parallax. — Ordinary students of 

 nature who are not experts in astronomy, must have 

 been sorely confused by the recent investigations of 

 this subject. Beginning their intellectual life with 

 the school-book statement of ninety-five millions of 

 miles as our mean distance from the sun, which was 

 Captain Cook's measurement and venerated accord- 

 ingly, they were next informed that the results of 

 the modern, and of course much better, observations 

 of the transit of Venus in 1882 reduced this distance 

 to 91,500,000 miles, and that all the greater celestial 

 distances, as well as the velocity of light, must be 

 reduced in like proportion. This lasted for only a 

 short time when other variations were supplied by 

 Mr. Gill's heliometer observations of the opposition 

 of Mars in 1877. This brought up the distance to 

 somewhat more than ninety-two millions of miles. 

 Since that we have the measurements of the velocity 

 of light, which bring it up to ninety-three millions. 

 It remains to be seen whether we shall go on in this 

 direction until we return to the old-fashioned ninety- 

 five millions of our schooldays. 



