The problem of the moving air mass, however, is more

complicated than it looks
The problem of the moving air mass, however, is more
complicated than it looks. For with the air is mixed a quantity
of water vapor. In a strict sense they are independent
variables, and the view set forth in most text-books that air
has a certain capacity for water vapor is misleading. We seldom
meet with pure, dry air. A cubic meter of such a gas mixture
would weigh 1,247 grams, at a temperature of 283 degrees A. (50
degrees F.). If chilled ten degrees, that is, to the freezing
point of water, it would weigh 46 grams more. So that by
cooling, air becomes denser and heavier. A cubic meter of a
mixture of air and water vapor at saturation, at the first
temperature above mentioned weighs only 1,242 grams, or five
grams less, and if this were cooled ten degrees the mixture
would weigh three grams less than the same volume of pure dry
air. We see that in each case the mixture of air and water
vapor weighs less than the air by itself. One would think that
by adding water vapor which, while light, still has weight, the
total weight would be the sum of both. It really is so,
notwithstanding the above figures, and the explanation of the
puzzle is that there was an increase in pressure with
expansion, so that the volume of the air and saturated vapor
was greater than one cubic meter. Since then a cubic meter of
air and saturated vapor weighs less than a cubic meter of dry
air at freezing temperature, speaking generally, we may expect
moist air to rise and dry air to fall. Consequently, if in
addition to falling temperature there is also a drying of the
air, we shall have an accelerated settling or falling of cold
dry air to the ground, which of course favors the formation of
frost. The water vapor plays also another role besides that of
varying the weight per unit volume. The heat received by the
ground consists of waves of a certain wavelength; but the heat
re-radiated by the ground consists of waves of longer
wave-length, and these so-called long waves (12 thousandths of
a millimeter) are readily absorbed by water vapor. Thus water
vapor acts like a blanket and holds the heat, preventing loss
of heat by radiation to space. Further on we shall speak of the
high specific heat of both water and water vapor as compared
with air and show the bearing of this in frost fighting; but at
present we may from what precedes formulate the second law of
frost fighting as follows: 'Frost is more likely to occur where
the air is dry than where it is moist.' It is also true that a
dusty atmosphere is less favorable for frost than a dust-free
atmosphere. Thus we may generalize and say that whatever favors
clear, still, dry air favors frost. The theory of successful
frost fighting then is to interfere with or prevent these
processes which as we have seen facilitate cooling close to the
ground. In what way can this best be done?

A brief glance at this table will show how easily one might slowly

starve on very expensive food, and yet how easily the energy food needed
can be secured at a very low cost
A brief glance at this table will show how easily one might slowly
starve on very expensive food, and yet how easily the energy food needed
can be secured at a very low cost.

I will hazard the opinion that the practice of establishing libraries in

towns for the benefit of a portion of the inhabitants only is likely to
prove pernicious in the end
I will hazard the opinion that the practice of establishing libraries in
towns for the benefit of a portion of the inhabitants only is likely to
prove pernicious in the end. To be sure, reading for some is better than
reading for none; but reading for all is better than either. In
Massachusetts there is a general law that permits cities and towns to
raise money for the support of libraries; yet the legislature, in a few
cases, has granted charters to library associations. With due deference,
it may very well be suggested, that, where a spirit exists which leads a
few individuals to ask for a charter, it would be better to turn this
spirit into a public channel, that all might enjoy its benefits. And it
will happen, generally, that the establishment of a public library will
be less expensive to the friends of the movement, and the advantages
will be greater; while there will be an additional satisfaction in the
good conferred upon others.

The "Observations on Man" (1749) is the first systematic effort to

explain the phenomena of mind by the Law of Association
The "Observations on Man" (1749) is the first systematic effort to
explain the phenomena of mind by the Law of Association. It contains
also a philosophical hypothesis, that mental states are produced by the
_vibration_ of infinitesimal particles of the nerves. This analogy,
borrowed from the undulations of the hypothetical substance aether, has
been censured as crude, and has been entirely superseded. But, although
an imperfect analogy, it nevertheless kept constantly before the mind
of Hartley the double aspect of all mental phenomena, thus preventing
erroneous explanations, and often suggesting correct ones. In this
respect, Aristotle and Hobbes are the only persons that can be named as
equally fortunate.

Ordinarily cold air falls to the ground; but not always, for

under certain conditions cold, heavy air may actually rise,
displacing warm, lighter air
Ordinarily cold air falls to the ground; but not always, for
under certain conditions cold, heavy air may actually rise,
displacing warm, lighter air. But such conditions can be
explained and there is no contradiction of the fundamental law
that if acted on only by gravity, cold air, being denser, will
settle to the ground and warm air, being lighter, will rise.
And there must be a certain relation between the height of the
level from which the cold air falls and the level to which the
warm air rises. In other words, we have to apply the laws of
falling bodies since a given mass of air, although invisible,
is matter and as subject to gravity as a cannon ball.