xil INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1847 To 1855. 
Now 
hol Hea 108 Belenet ame (Introduetion, 1845-46, p, xxxi.), 
= 1:08 x 0:00032675 x cot 70° 36’ = 0:0001243, 
which therefore represents the value of one scale division in parts of force after 
the change. 
Hence, also, 
which therefore represents. the temperature correction for 1° Fahr. in scale divi- 
sions after the change. 
In order to make the reading 500 represent the same value of force after 
the change as before it, a process similar to that given pagina 1845-46, 
p. Xxxii.) must be adopted. 
Thus we have reading before change corrected for temperature + 340 = 595-4. 
Reading after change corrected for temperature + 340 = 516°9. 
Hence 
f (595°4 — 500) 0:000135 = (516-9 — z) 0:0001243, 
whence | 
2= 4132; 
the difference between which and 500 is 86°8. Adding 86°8 to 340:0, we obtain 
426°8. 
If, therefore, we use 426°8 after the change to replace 340 before it, the same 
zero, 500, will be applicable both before and after, and consequently the tempera- 
ture tables after the change will be formed thus :— 
Let N denote the observed reading, 
t the temperature Fahr., _ 
n the reading corrected by means of the temperature tables ; 
then 
n =N + 4268 + (¢ — 26°) 2:14. (See Introduction, 1845-46, p. XXXlil,) 
Also the means f in parts of the whole horizontal force, given in the abstracts of 
results, are obtained after the change by the formula 
f = (n — 500) 0-0001248, 
the corresponding formula before the change being 
= (n— 500) 0:000135. E . (Introduction, 1845-46, p. xxxiii.) 
BALANCE OR VERTICAL FoRcE MAGNETOMETER. 
A detailed description of this instrument, and of the method of observing it, 
and reducing the observations, will be found in the Introduction, 1845-46, p. xxxiii. 
The object of the instrument is to measure the variations of the vertical com- 
ponent of the earth’s magnetic force. It consists of a magnetic needle balanced 

