412 
(divided on glass) are seen reflected; one from the middle mirror of 
the weight, the other from the ring mirror at the same time. Such 
scales have been employed by the author with small sextant tele- 
scopes, for which one division equalled twenty seconds, and two 
seconds could be estimated. 
Let us now suppose the two mirrors parallel, the spring exerting 
no action on the weight, and that the middle mirror faces the south. 
The observation commences by turning the ring (say directly) through 
an exact revolution, which is known by the mirror showing exactly 
the same scale division. The eastern mirror, which makes an angle of 
about 86° with the middle mirror, will now bring the scale into the 
field of the telescope (if the proper precautions have been taken by 
the maker, in adjusting the force of the spring and the interval of 
the wires to each other) ; and the scale division being made, the exact 
angle (y) through which the weight has been turned will be obtained. 
The ring should now be returned to its first position, and the obser- 
vation repeated by turning the ring in the opposite direction (say 
retrograde) through a revolution. 
It is evident that it is not necessary that the ring should be turned 
exactly through a revolution ; it may be turned exactly a few degrees 
more or less by means of the scale, so that observations may be made 
with different values of v } and therefore with greater or less delicacy. 
Thus, as may easily be shown, when i> = 86°, a variation in height 
of ten feet will be equivalent to a variation of three seconds ; when 
^ = 88°, the variation of three seconds will be produced by a rise of 
about five feet only. In proceeding from the poles to the equator, 
v should vary from 84° to 90° nearly. 
The precautions taken to render the instrument portable, so as to 
keep the spring and wires with constant forces, the means of correct- 
ing for the effect of temperature, &c., need not be noticed till the 
completion of the instrument. 
It should be pointed out as a great advantage of the instrument, 
that a large weight is not required, since it is the ratio of the varia- 
tion of weight to the whole weight which is shown. 
5. On the Law of Growth in Woody Circles of Exogenous 
Trees, as indicated by the Examination of a Single Speci- 
men. By Principal Forbes. 
