Lovering and Bond on Magnetic Observations at Cambridge. 65 
Supposing that the inclination has been on the increase since 1782, 
its mean annual rate would be equal to 4.’5. It is thought that the 
inclination is decreasing at present in the United States as it has 
long been in Great Britain and on the continent. Professor Loomis 
assigns the rate for this country at 1.8 yearly. If this be so, we 
observe the same great difference in the rate of increase and de- 
crease of the inclination as of the declination. In either case it is 
equally inexplicable. The want is now felt of systematic observa- 
tions of the dip in order to eliminate regular and irregular changes 
of this element and obtain a true mean for a given period. The 
general scheme of magnetic observations has not failed to provide for 
this, as we shall see further on. So far as the subject has been in- 
vestigated, the dip does not appear to pass through such a uniform 
succession of positions daily as the declination. Its value often 
changes suddenly and to the amount of a degree and a half although 
according to Kupffer the regular daily range does not exceed five 
minutes. As the various observations on the Dip at Cambridge and 
Dorchester were made with the same needles, the range of 21’ in 
the values cannot be charged to the needles, and it is too great to 
come under the daily or annual variation. Observations made else- 
where show that such irregular disturbances occasionally derange the 
dip. Moreover the mean value, derived from frequent partial results 
obtained on different days and different hours of the same day, is 
still exposed to what may be called a constant error of the needle 
and which is not eluded by any of the different reversals. Thus 
out of 8 needles used by Captain James Ross in London, 2 differed 
41’ inthe values which they gave for the dip; although from 640 to 
1000 readings were made with each. We subjoin the following 
Table drawn up by Quetelet,* which contains the value of the an- 
nual diminution of the magnetic inclination at various places : 
* Nouveaux Mémoires de l’Académie Royale des Sciences et Belles-Lettres 
de Bruxelles. Tome XII. 1839. 
9 
