TRANSACTIONS OF SECTION A. 



589 



longitude co-ordinates, referred to the mean station, and measured in minutes of 

 arc° and 6, H, F, S being respectively the dip, the horizontal force, the total force, 

 and the declination. 



6 =50°28'^6 + (M41<^-^1556Xy 



H = •29482 - -0000617 ^ - •0000117 X 



E = ^46407 + 000094 ^ - ^000045 X 



8=4° 63'^3 + (^241 ^ - ^109 X - •000231 X^)' 



The horizontal and total forces are measured in C.G.S. electro-magnetic units, 

 and the declination is west. 



From these expressions we may find the quantities u and r for the mean 

 station : — m being tbe angle between a given iso-magnetic line drawn eastward, and 

 the lono-itude line drawn northward, and r being the rate of change of the given 

 element per kilometre of distance measured in a direction perpendicular to the iso- 

 magnetic line. The values of u and r for the several quantities are given in the 

 subjoined table, together with the value of the element at the mean station : — 



Regarding the secular changes in the elements very little can be definitely said. 

 If we admit the accuracy of Ino's observation of eighty years ago, that the needle 

 pointed true geographical north at that time, then the mean secular change during 

 these eighty years will be about '6'-7 per annum. Gauss's numbers give 1° 46' W. 

 as the value of the declination at the mean station of the present survey. Taking 

 fiifty years as the interval between Gauss's epoch and now, we get again about 3'^7 

 as the mean annual rate of change of the declination in Japan. Recent observa- 

 tions, however, do not bear out this conclusion. Thus, if we compare (as far as 

 comparison is possible) the observations made in the summer of 1887 with those 

 made during the autumn and winter months of 1 882 and 1883 by Messrs. Sekino 

 and Kodari at their numerous but not very well distributed stations, we find no 

 evidence of any well-marked secular change of the declination during the interval 

 in question. There are indications, however, of changes in the other elements, 

 namely, an average decrease of 2' per annum in the dip, an increase of nearly 0^1 

 per cent, in the horizontal force, and a decrease of fully O'l per cent, in the total 

 force. 



The full details of the survey are given in the second volume of the ' Journal 

 of the College of Science, Imperial University, Japan.' 



8. 



On Reading Electrically Meteorological Instruments distant from 

 the Olserver'} By J. Jolt, M.A., B.E. 



An instrument placed in a distant observatory may on this system be con- 

 trolled by three wires communicating -with the home station. The principle on 

 which a reading is obtained is, briefly, as follows. The instrument is in circuit 

 with a rheotome at the home station. One contact on the rheotome corresponds 

 to a movement over a definite distance of a travelling limb on the distant instru- 

 ment, the limb being actuated by an electro-magnet. In the case of the barometer 

 or thermometer, for example, the travelling limb carries a metallic pin through a 

 definite distance at each make, starting from a known zero position, till finally the 

 pin makes contact wiih the mercury in the instrument, when a needle in the home 

 station is deflected. The position or level of the mercury is thus easily computed 

 from the reading of the rheotome or number of interruptions efiected. The 



> Proc. Royal Dublin Society, vol. iv. new series, p. 158. 



