846

THEORY AND PRACTICE OP ENGINEERING.

Rook II.

pressure ; if we therefore know the law bywhich the altitudes increase, it is possibleto determine the difference between theelevation of two places when their meanbarometrical altitude are given; it beingan established law of nature, that the pres-sure of the atmosphere decreases in ageometrical progression, as the height ofthe place of observation increases in anarithmetical progression. To measurethe height of any place, the altitude ofthe mercury must be measured simul-taneously at both the lower and upperstations; the logarithms of these twobarometric altitudes being found, the lessis to be subtracted from the greater ; thedifference will be proportional to thedifference of the elevation of the twoplaces: to find the real height, this loga-rithmic difference must be multiplied bya constant number found by previous ex-periments. Professor Littron, in theTransactions of the Astronomical Society,has given the necessary formula; andtables, by which great exactness may bearrived at in the measurement of heights ;but the system is somewhat complicated,and needs great care, accurate observation,and considerable calculation.

The Chain and Offset Staff are em-ployed alone in land-surveying, or in con-junction with the theodolite, or otherinstrument for measuring angles: whenthe chain alone is employed, no otherfigure can be used than the triangle,and the correctness of the survey dependsupon the accuracy with which its sidesare measured. The length of the chainmust be accurately tested to ensure cor-rectness, before any of the distances aretaken ; to do this it must he stretched ona level piece of ground, and with an ac-curate rod it must be carefully measured,and if any error is apparent, it must berectified equally on both sides of the cen-tre : after this has been done, it may beapplied upon a wall, and a permanentmark made upon it, which will serve atany future time to compare it with ; longchains enable us to measure with greateraccuracy than shorter ones, and anglesformed by them are less obtuse.

Gunter's Chain , as it is called, has beenfound the most convenient where thecomputation of acres is required, and is ingeneral use by land-surveyors; its lengthis 66 feet, and it is divided into 100 links:ten of these square chains equal an acre,and as the chain is divided into 100, thecontents expressed in chains and links areconverted into acres and decimals of anacre, by dividing by 10; an acre beingequal to 66 feet multiplied by 66 feet, andthen again multiplied by 10, 43,560 feetbeing equal to 100,000squarelinks. The

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R, R, section,n, section of glass.

Fig. 1072.

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rood is one fourth of an acre, and the rod or perch one fortieth of a rood; thus, after re-ducing the area to square links by cutting off the last five figures, we obtain the acre* ir»