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THE METEOROLOGY AND CLIMATE OF PLYMOUTH. 425 
of 24 inches of mercury. It is but seldom the pressure here falls 
below 29 inches, this having occurred only forty-eight times in 
the sixteen years. The month of greatest mean pressure is June, 
the average being 30°022 ; the month of least pressure is October, 
the average being 29°888. These averages form no guide to the 
months of greatest extremes, which mostly concern us as residents 
in a maritime town, because in these months we get the strongest 
winds, and they therefore bring disasters to our relatives and 
friends who go down in ships on the great waters. These months 
are the winter months of December (1°368), January (1°412), 
and February (1°303). Our average pressure for the year is 
29°945 inches. 
TEMPERATURE.—By temperature I mean the heat shown by an 
exposed thermometer. In Plymouth we have few of the extremes 
of heat and cold which places more inland experience. The 
chief reasons are the following: Maritime places on western 
seaboards, if situate in the Temperate Zones, are in the direct path 
of the Return Trade Winds. These winds have been heated in 
the Torrid Zone, and there ascend, giving up their heat to limitless 
space; and in their ceaseless journey towards the poles again 
descend in about 30° or 35° of latitude, but have a westerly ten- 
dency given to them because they come from a portion of the earth 
where the daily rotation is much quicker than at places nearer the 
poles. Hence they blow from the south-west in north latitude, and 
from the north-west in south latitude. But before they reach us 
they pass for thousands of miles over the ocean, and this (from the 
great specific heat of water) again makes them hot, and thus they 
reach us as warm, wet, south-west. winds. By specific heat I un- 
derstand the amount of heat necessary to raise a unit of a substance 
a unit in temperature. This may be better understood thus: Let 
us take a pound of water, and also a pound of mercury at the 
same temperature, and similarly expose them to the same source of 
heat, as a steam-chest or sand-bath. We shall find that although 
we have equal masses of the two substances, yet the mercury will 
get hot the quicker, and will rise in temperature a certain number 
of degrees, thirty times as quickly as the water will. Now because 
the two substances are similarly exposed, it is evident there must 
be thirty times more heat absorbed by the water than by the mer- 
cury to produce the same thermal effects. Water, we say, has 
