40 SURVEY OF MASSACHUSETTS. 
ago without any apparent derangement, and I presume will continue to stand and retain 
their adjustment until they decay. Before I dismiss this subject I would further men- 
tion, that whenever our stations happened to be situated where rocks were not to be found, 
as upon Cape Cod, Nantucket, &c., we marked the stations by burying a bottle about 
eighteen inches beneath the surface, first filling it with sand, so that it should not collapse 
if it should happen to be broken. 
x Fic, 3, Fig. 3. Shows the position of the signal while we are measuring angles 
\ ~ atthe station. A sketch of the tent which protects the theodolite, is seen 
$ suspended from the frame. 
Fig. 4. Shows the position of the signal when we put new cloth or a 
tuft of pine or other species of boughs upon it, or make any repairs, addi- 
tions or alterations connected with the top of the signal staff. 
When the signal frames have been light or slender, we have bored holes 
into the lower ends of the braces and inserted strong pins, which we have 
loaded with stones to prevent the wind from blowing them down. And 
where stones were not to be found, we have driven strong pins into the 
earth, near the ends of the braces, and nearly at right angles therewith, 
which we have nailed firmly to the frame. 
.. Respecting atmospheric phenomena, I would state that I have rarely 
been able to measure angles with a satisfactory degree of accuracy, when- 
ever objects appeared in a striking degree looming up, as the sailors call it. ‘These loom- 
ings or mirages are frequently seen, particularly upon the sea coast. I have often 
seen portions of the tops of mountains, valleys, trees, and houses double, that is, one above 
another. Sometimes I have seen objects triple, but I have never seen their images inverted. 
My experience shows that objects are refracted side-ways during extraordinary mirages; 
and I am very confident that they are frequently laterally refracted when there is no ap- 
parent mirage. 
As respects the transparency of the atmosphere, or more correctly speaking that state 
of the atmosphere which permits distant objects to be distinctly seen, I have made many 
observations, with the view of ascertaining the reason why distant objects cannot be dis- 
tinctly seen on certain days when the ‘atmosphere appears to be uncommonly transpa- 
rent. As Iam not aware that any person has published an account of this apparent 
paradox, I will endeavour to state my experience upon this subject in as brief a manner 
as Lwell can. Hints of this kind may prompt others to investigate the subject; and whe- 
ther my theory be found true or false, will be of little importance, provided the real 
cause be discovered, and we be enabled to predict correctly the state of the atmosphere, 
with respect to seeing distant objects distinctly. ‘To the engineer engaged in trigonome- 
trical operations this knowledge will be of great value, as it may save him the trouble of 
much useless and vexatious labour. It may also be useful at times to those engineers 
who have never been engaged in very extensive surveys, and of course can know but 
little from their own experience of the different states of the atmosphere; particularly 
when they have occasion to run a line of considerable extent, or wish to observe very dis- 
tant objects. The curious traveller might also add much to his gratification, could he be 
