286 OBSERVATIONS ON THE MAGNETIC DIP, 
reversals. The hour of observing the dip is not put down in the tables, but it may in 
general be understood to have been about thirty minutes previous to the commencing of 
the observations for determining the intensity of force. The intensity apparatus was of 
the model invented by Professor Bache, by means of which Hanstenian needles are 
vibrated in a glass vessel nearly exhausted of air. 
In almost every case these vibrations were commenced with an arc of about seven 
degrees on each side of the magnetical meridian, and continued to the number of five hun- 
dred, the arc then being between one and two degrees. The temperature was cur- 
rently noted by means of an interior thermometer, and the duration of the vibrations 
reduced to the standard temperature of 60° Fahrenheit. The constant co-efficient for 
the effect of temperature on each needle, was determined by experiments in a room in the 
first place at the temperature of about 40°, and next in the same room artificially heated 
by a copper stove to near 90°. The result of these experiments was that the co-effi- 
cients for the several needles were as follows: 
No. 1. .00012 No. 4. .000065 
« 2. 000145 “ 5. .000070 
“ 3. 000058 “ 6. .000088 
Needle No. 1, made by Robinson, was found to be somewhat inconstant in its magnet- 
ism, as will be seen by the observations recorded in the tables, some slight changes are 
also apparent by the results by No. 2, but Nos. 4, 5, and 6, made by myself, have proved 
themselves to be of superior quality. ‘They were made of watch-makers’ steel wire, and, 
were left at the highest degree of hardness which could be given to them by fire and 
water. 
The chronometer used, made by Molyneux, was in all cases kept running so near to 
the mean rate that no equation was necessary except in the expedition to Lake Superior in 
1843, when its daily rate was found to be fourteen seconds too fast. I regret that it was 
not always in my power to determine with more precision the magnetical declination, 
which I observed in but few instances, and then only by means of plumb-lines and naked 
sights, in the ordinary manner of the United States surveyors. 
Explanation of the following Tables.—At the head of each is placed the date and the 
latitude and longitude of the place, either from the best observations, or from the most 
approved maps, ‘l’anner’s being, for the most part, preferred. In the Iowa and Wiscon- 
sin region, Judson’s map was used, and, in the region of Lake Superior, Captain Bay- 
field’s, as published by the Society for the Promotion of Useful Knowledge. 
In the first column is placed the dip, as determined by the mean of sixteen observa- 
tions; in the second, the number designating the intensity needle; in the third, the epoch 
of commencing the vibrations; in the fourth the observed duration of five hundred vibra- 
tions; in the fifth, the mean temperature, as indicated by an interior thermometer; in 
the sixth, the calculated duration at the standard temperature of 60°; in the seventh, the 
square of the number found in the sixth column; in the eighth, the horizontal intensity, 
that at Cincinnati, at some specified date, being reckoned 1000; in the ninth, the total 
intensity of force, in terms of the above named horizontal intensity at Cincinnati ; in the 
tenth, the ToTAL INTENSITY OF FORCE, that at Cincinnati being recorded 1000. In the 
