That is the correct scansion for that line - the poem is written in dactylic hexameter - and the stressed and unstressed syllables don't alternate. The patterns of stressed and unstressed syllables alternate, but in this case, the pattern is X / /, not X / or / X. The best guideline is to say the line normally and be patient.
An isobar refers to all of the nuclides of all elements that have a particular atomic mass. For example, helium-6 is one of the many isotopes of helium. The isobars of helium-6 are hydrogen-6, lithium-6, beryllium-6, and boron-6.
Points of equal pressure can be connected with a line, and these are the isobars. They are commonly shown on a map in increments of four millibars. Isobars drawn on a map this way make it visually easier for a meteorologist to interpret. One way to think about an isobar map is like it's showing elevation. Areas of high pressure are like mountains, and areas of low pressure are like valleys.