THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 159 



not been sufficiently multiplied to enable us to state with certainty the 

 extent and manner of this variation for New England. Cassini began 

 daily observations in Paris, in 1783, by which, in 1786, he was able to 

 announce the discovery of this annual variation. By his observations it 

 appeared that the westerly declination increased from June 20 to March 

 20, and from March 20 to June 20, decreased by about one third of the 

 increase from June to March. Subsequent observations in other places 

 do not fully confirm the results obtained by Cassini. It is more probable 

 that there are two periods of retrogression, one between the vernal 

 equinox and summer solstice, and the other between the autumnal equi- 

 nox and winter solstice. This seems to be indicated by the observations 

 of Gilpin, about the beginning of the present century, in England. But 

 it is not necessary for our present purpose to pursue this part of the 

 subject further, as this variation in New England is too small to require 

 notice in the use of the needle, being probably less than one minute. 



5. Magnetic Storms. Those irregular and occasional disturbances in 

 terrestrial magnetism called magnetic storms, are generally attended by 

 an aurora, and no doubt are one effect of the same cause which produces 

 the aurora. They occur by day as well as by night, and therefore are 

 not always accompanied by a visible aurora. Their duration and the 

 amount of disturbance they produce are as varied as the features of our 

 rain-storms. During a magnetic storm the needle is observed to be 

 unsteady and tremulous, changing its direction, now this way and now 

 that, to an extent dependent on the magnitude of the storm. Slight 

 disturbances of this kind, affecting the direction of the needle by a few 

 minutes, are not uncommon. The diurnal curve for January 9, 1872, 

 shows such a disturbance. This disturbance continued through the 

 night, beyond the limits of this diagram ; and, as before stated, for 

 several days after, the average pointing of the needle was some 10' 

 farther east than usual. There were also many other days of unusual 

 disturbance during this month ; and on the 4th of the following month a 

 most remarkable storm was observed by the writer and his assistants at 

 the Dartmouth College observatory. The day was cold and windy, the 

 weather clearing after a heavy fall of snow. In the evening appeared 

 that most remarkable aurora, covering the whole heavens south as well 

 as north, which many will remember, and which was seen in Europe as 



