arly--but does not show the temperature of mercury within exactly. It does so however near enough for ordinary practical purposes--provided that no sun, nor fire, nor lamp heat is allowed to act on the instrument partially.
The mercury in the cistern and tube being affected by cold or heat, makes it advisable to consider this when endeavouring to foretell coming weather by the length of the column.
* * *
Briefly, the barometer shows weight or pressure of the air; the thermometer--heat and cold, or temperature; and the wet thermometer, compared with a dry one, the degree of moisture or dampness.
It should be remembered that the state of the air foretells, rather than shows present weather (an invaluable fact too often overlooked); that the longer the time between the signs and the change foretold by them, the longer such altered weather will last; and, on the contrary, the less the time between a warning and a change, the shorter will be the continuance of such foretold
For history buffs, Fitzroy was the captain of the Beagle, which took Charles Darwin on the 5 year cruise that lead eventually to the Theory of Evolution. Fitzroy was an early advocate of weather forecasting - for which, of course, he was ridiculed in the British Press.
The first part of the book is the author's plea for the coast guard stations and all ships to have a barometer to avoid bad weather and shipwrecks. They had a lot of boats unknowingly going out while storms were brewing, with loss of lives and property.
The rest of the book is invaluable for amateurs who want to forecast the weather themselves. It explains how a barometer works and how to forecast based on barometer readings, wind direction, and cloud signs.