. The cyclopaedia; or, Universal dictionary of arts, sciences, and literature. Encyclopedias and dictionaries. COMPASS. gooths of an inch. It is fufFicient if they are nearly fo, as, for jnftance, from about 2S0 to 320 in an inch ; becaufe by pull- ing the tube T more or lefs out of the tube SS, and by moving the lall mentioned tube up or down into P, the divifions of the circle may be magnified more or lefs, till 15 divifions of the micrometer appear to be exactly fqual to the fpace between any two contiguous divifions on the circle; viz. to half a degree. When the microfcope has been once fo
. The cyclopaedia; or, Universal dictionary of arts, sciences, and literature. Encyclopedias and dictionaries. COMPASS. gooths of an inch. It is fufFicient if they are nearly fo, as, for jnftance, from about 2S0 to 320 in an inch ; becaufe by pull- ing the tube T more or lefs out of the tube SS, and by moving the lall mentioned tube up or down into P, the divifions of the circle may be magnified more or lefs, till 15 divifions of the micrometer appear to be exactly fqual to the fpace between any two contiguous divifions on the circle; viz. to half a degree. When the microfcope has been once fo adjufted, it requires no farther alteration, excepting to be moved round the box until it comes over the end of the needle, in which fituation the field of view ap- pears as (hewn in^g. 22, where the middlemolt part is oc- cupied by the micrometers 1,2,3,4.; the other large divi, fions arc the magnified divifions of the circle, and W is the extremity of the needle, with its directive line pointing to fome diviiion on the ; " Fifteen divifions of the micrometer being equal to half a degree, or to 30', it follows that each divifion is equal to two minntes, and as it may be ealily difcerned whether the line on the end of the needle is directed exactly to one of thofe divifions or midway between two of them, therefore the direction of the needle in this compafs may be read off to a (ingle minute. In order to obferve the daily variation, when this compafs is properly fixed, the microfcope njed not be moveu, becaufe its field of view takes in about two degrees, which is a much greater fpace than the needle will go through in ten years time. In moving the microlcope round the box, care muft be taken to place it fo that the divifions of the circle may appear to coincide with fome of the long divifions of the micrometer N. B. This microfcope inverts the objects ; therefore what appears to be on the right hand mull be underftood to lie on the left, and -vice •oerf
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Keywords: ., bookcentury1800, bookdecade1810, booksubjectencyclo, bookyear1819
