CURRENT INDUSTRIAL NE.W5

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Figs. 3 and 4. The instrument is essentially a gas-balance, in which the specific gravity of the gas under test is compared with th at of the surrounding air. This is accomplished differ­

entially, the weight of a column of gas and the weight of an equal column of air being arranged to act on opposite sides of w hat is virtually a scale-pan, and the difference in weight reg­

istered.

T he bell of the gravitometer m ay be considered as a scale- pan on which the pressure below is due to the pressure of the atmosphere on the lower surface, and the pressure above to the pressure of a column of gas added to the pressure of the atmos­

phere at the top of th at column, the aggregate being less, in the case of coal-gas, than that of the atmospheric pressure below the bell. Gas is always passing up the tube to supply the burner a t the top, and under these conditions is under absolutely the same conditions of temperature and pressure as th at of the sur­

rounding air. It will thus be seen th at the pointer of the gravi­

tometer indicates the weight of that column of gas of a height equal to th at of the tube. For example, with a bell of 6 in. di­

ameter and a height of tube of 30 in., the gas of, say, 0.9 specific gravity is 15.9 grams.

The pressure or weight on the top of the bell is in direct pro­

portion to the specific gravity of the gas in the tube, and is, in addition, proportional to the height of the tube. Thus, if the tube were 6o in. in height instead of 30 in., the pressure on the bell, due to the weight of the gas, would be 30.38 grams, and the movement of the pointer across the scale would be twice th at in the case of the 30-in. tube. Mechanical considera­

tions constitute the only reason for not using a longer scale;

indeed, b y employing tubes of sufficient height, it is possible to measure specific gravity to any required degree of accuracy.

Referring to Fig. 3, which shows diagrammatically the action of the gravitometer, A represents a light aluminium bell, sealed in oil contained in the annular tank B . The Bell is covered by a shell, C, which is not in contact with it. The top of the shell carries a tube, D . The bell is free to move up and down, and is carried b y a vertical support connected to the end of the bal­

ance-beam G b y a hanging chain. T he other end of the beam carries a hanging weight, H. The chains connected to both H and A pass over circular arcs. A pointer, K , and a gravity- control weight, L , complete the essentials of the instrument.

On a stream of gas passing slowly over the bell A and up the tube D , the pressure on the top of the bell is diminished and the pointer K swings into a new position of equilibrium. The scale over which the pointer moves is graduated to read or to record specific gravity directly.

T H E IM PE R M E A B IL IT Y OF C O N CR ETE

In a paper recently read before the Western Society of Engi­

neers and quoted in Engineering (London), 98 (1914)» 483.

Professor M . O. W ithey described the results of a series of tests on the permeability of concrete, which have been made a t the U niversity of Wisconsin. T he materials used were Portland cement, sand ranging in weight from 104.5 lbs. to 112.2 lbs. per cu. ft., and gravel weighing from 107.3 lbs. to 190.3 lbs. per cu. ft.

E ighty-eight of the test-pieces were made with a 1 : i ’/s ■ 3 mixture b y volume, and sixty-seven with a 1 : 2 : 4 mixture, and there were ninety-eight specimens proportioned with 1 part b y weight of concrete to 9 parts b y weight of aggregate. None of the concretes proved absolutely water-tight in the sense th at they would not absorb water, but most were so impervious th at there was no visible evidence of flow. The signs of damp­

ness on the bottom of the specimens increased with increasing hum idity of the atmosphere. W ith mixture of 1 part of cement to 7 parts of aggregate the average rate of flow during a period of fifty hours was under 0.001 gal. per sq. ft. per hr., when the pressure was 40 lbs. per sq. in. W ith the 1 to 9 mixtures, prac­

tically water-tight concrete could, it was found, be obtained by suitably grading the sand and gravel. Richer mixtures, such as the 1 : 1V2 : 3, proved very impervious, but Professor W ithey remarks th at such rich mixtures show considerable volume changes when alternately wetted and dried. T o secure imper­

m eability great care is needed in mixing the concrete, especially when the proportion of cement is small. If mixed too dry, the concrete cannot be properly compacted. The best results were obtained b y mixing the materials dry for l/i to */j min., and then continuing the process after adding the w ater for 1V2 to 2 min. with 1 to 9 concrete, or for 1 min. with the rich 1 : i V j : 3 mixture. Proper curing of the concrete greatly adds to its impermeability. Prem ature drying destroys the imperviousness of the lean mixture, seriously impairs that of the 1 : 2 : 4 mixture, and appreciably diminishes the water­

tightness of the rich mixture. Thus, thin sections of 6 in.

to 8 in. in thickness should be, he concludes, kept damp for one month for lean mixtures, or two weeks for a rich one.

Engineering also quotes (vol. cit., 446) from Science Conspectus, a publication of the Society of Arts of the M assachusetts Insti­

tute of Technology, some particulars of an interesting series of experiments now being carried out by the Aberthaw Construc­

tion Company in order to disprove the theory th at the combined effects of sea-water and frost rapidly destroy concrete struc­

tures.

W ith this object in view, twenty-four concrete columns, 16 ft. long, 16 in. square, and reinforced w ith bars near the comers, were constructed in January, 1909, and immersed in the water at Boston N a v y Y ard . T h ey were suspended in such a manner that at high tide the water reaches nearly to the top of thc column, and falls at low tide nearly to the bottom. In cold weather the columns are thus alternately thawed and frozen as the tide rises and falls. T he columns were made with various qualities of concrete mixed dry, plastic, and very wet. Differ­

ent qualities of cement were used, and the effects of waterproofing materials, clay, and other additions to the concrete are being studied. One of the columns was mixed with salt water, but this was unfortunately lost in handling.

N o final conclusions are, of course, possible yet; m any years must, in fact, elapse before it will be possible to say which kind of concrete is most permanent. When examined in December last m any of the specimens were practically unaffected, but others were badly eroded. A s might be expected, the best results were given b y the specimens richest in cement and mixed wet. For instance, of two columns made with 1 part of cement to 1 of sand and 2 of stone, th? one mixed dry was badly eroded over the whole of its length; whereas the other, which was mixed very w et,-w as only slightly pitted. Again, of two specimens made with slag cement in the ratios of 1 : 1 : 2 and 1 : 3 : 6 , respectively, and both mixed wet, the former was in excellent condition, with only very slight pitting, while in the latter all thc corners had gone and the reinforcement was exposed in places.

The part of this specimen which was continuously immersed was, however, in very fair condition. The experiments are being continued, and doubtless some very interesting results will be obtained in time.

P U T T IN G E L E C T R O T Y P IN G IN D U ST R Y O N M O R E SC IE N ­ TIFIC B A SIS

A t a meeting in New Y o rk on October 7, 1914, the Inter­

national Association of Electrotypers appointed a committee to cooperate with the Bureau of Standards in a study of the con­

ditions in the electrotyping industry, with a view to assisting in placing it upon a more scientific basis. A preliminary circular giving simple directions for testing and adjusting the density and the acidity of the copper electrotyping solutions has been prepared, and m ay be obtained upon request from the Bureau of Standards, Washington, D . C.

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