86 
ME. C. CHAMBERS ON THE DIRECTION AND INTENSITY 
yearly comparisons ; and secondly, that the probable error of a single weekly determina- 
tion (that is, of the mean of the first and second observation) being + -0043 (see 
paragraph 19), that of the mean difference, for six and a half years, between the first 
and second observations will be less than ±'0005 (or= + ‘0043-r- ( y/^|^X\/2), a 
quantity which is less than half the magnitude of the difference that the error is sup- 
posed to explain. And if it were due to erroneous allowance for temperature of the 
large Horizontal-force Magnetometer, it ought to be small when the diurnal range of 
temperature is small, and large when the range is large ; whilst, in fact, the difference in 
question is only -0003 in the half-year October to March, when the range of temperature 
is large, and is ‘0016 in the half-year April to September, when the range of tempera- 
ture is small. At the same time the range of the diurnal variation of Horizontal Force 
is nearly the same throughout the year, its value for the half-year October to March 
being a fourteenth greater than for the half-year April to September, and its mean value 
for the year, as shown by the large Horizontal-force Magnetometer, is - 00166 of the 
whole force. It remains, therefore, if the result be an instrumental one, that the scale- 
coefficient adopted for the large Horizontal-force Magnetometer must be supposed to 
he one fourth of itself in excess of the truth : this the writer cannot think possible, and 
he hopes soon to have the opportunity of submitting to the judgment of the Royal 
Society evidence (in connexion with a general discussion of the observations with this 
instrument) which will completely set aside such a supposition. 
If the result be admitted as a true magnetic phenomenon, it suggests the attribution 
of a very considerable magnetic influence to the state of the medium intervening between 
the upper and lower places of observation, in such a way that when the air is of equable 
temperature and almost uniformly moist throughout, the variations of force are nearly 
alike above and below, whilst in the dry months of the year there is a very considerable 
diminution of daily change of force with increase of height. The writer readily admits 
that such attribution should not rest upon a result of observations taken at a single 
station only. 
18. Annual Variation . — In Table XVII. are shown the means for each month, in the 
period of six and a half years, of the values of Horizontal Force, corrected to the mean 
monthly reading of the large Horizontal-force Magnetometer ; also the corrections for 
secular change, at the rate of -]- - 0045 per year, to reduce those means to the common 
epoch, October 1, 1870, and the same means cleared of secular variation ; and, 
further, the excess of each of the corrected monthly values above the mean value 
for the year, the last series of numbers representing the annual variation of Horizontal 
Force. 
