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.
A dramatic example on an isobar map is that of a hurricane. Isobar lines that show a hurricane appear in a tight circle and are very close together. This is because the wind is moving rapidly from the high pressure outside the storm to the low-pressure area of the eye of the hurricane, creating very strong winds.