42 SURVEY OF MASSACHUSETTS. 
water of the deep ocean in the course of a few hours.* The probability therefore is, that 
the surface of the water could not have been many degrees above freezing; and I suppose 
it must have been as low as 40°, if not lower. ‘The thermometer at the station, I find, by 
my memoranda, stood generally at 34°, and did not vacillate more than two degrees. The 
surface of the earth, as before stated, is supposed to have been of the temperature of 32°. 
These numbers will show that there was a very near agreement between the temperature 
of the air, and the temperature of the surface across which the observations were made. 
I came therefore to the conclusion that the nearer the atmosphere and the surface of the 
earth or water across which the line of sight lies are to the same temperature, the more 
distinctly can distant objects be seen in an atmosphere of equal transparency. Since the 
morning on which these observations were made, I have observed many phenomena, na- 
tural and artificial, which might be cited in corroboration of the above conclusion; and I 
have seldom seen any thing to disprove it. 
Before we finally dismiss this subject it may be well to describe, by way of elucidation, 
the different appearances of distant objects, commencing our description at a time when 
the objects appear plain and distinct, and continuing the description through all the dif- 
ferent grades of appearances until they have entirely faded away, or until they cannot be 
longer seen. 
Before entering upon the description, I would premise, that the atmosphere is supposed 
to be transparent; that is, free from fogs, smoke, dust, or other heterogeneous matter. 
In choosing a time to commence our observations, I have selected the morning, as it is 
not unusual that all the phenomena which I shall notice succeed each in the order of 
their description, We therefore commence in the morning of a favourable day. Let us 
then suppose that when the sun has risen sufficiently to illuminate the distant objects, they 
are seen distinctly. We have been led to the conclusion, that when distant objects are 
distinctly seen, the atmosphere and the surface over which our line of sight passes must 
be of the same temperature; and experience has shown us that soon after the sun rises, 
objects are generally more distinctly seen than at any other time of the day, previous to 
within an hour or two of the sun’s setting. And our reason will teach us that the sur- 
face of the earth and the atmosphere are oftener of the same temperature, morning and 
evening, than at other times of the day; therefore there must then be less of ascending and 
descending currents in the atmosphere. 
The objects at this time of the day (morning) appear perfectly still, and present a well 
defined and distinct outline. I have of course supposed them to be stationary. As the sun 
rises, the surface of the earth absorbs warmth more readily than the air, and of course the 
stratum of air which lies in immediate contact with the earth becomes rarefied and rises, 
forming ascending currents while the vacuum is supplied by descending currents. In 
consequence of this condition of the atmosphere, distant objects* present a waving ap- 
pearance ;—the light reflected from them appears to be sluggishly refracted from right to 
left and from left to right,—the image of the objects appearing of about the same dimen- 
sions as when first observed, and with still a distinct outline. The sun continues to rise:— 
the heat increases, and with it ascending and descending currents in the atmosphere;— 
the images of the objects lose their wavy appearance, and appear larger and fainter ;— 
* In support of this conclusion I would state the fact, that the ocean frequently freezes in the Vineyard Sound and at 
Nantucket, so extensively, that no water can be seen for many successive days. 
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