ARTICLE X. 
NOTES ON CERTAIN MODES OF MEASURING MINUTE INTERVALS OF TIME. 
BY J. ©. ADAMSON, D. D. 
[Read, February 6, 1857.] 
It has become necessary to devise means of estimating minute intervals of time for two 
purposes. Of these purposes, one is, to ascertain the absolute length of an interval merely, 
or the absolute duration of a transient phenomenon, such as the passing of an electric cur- 
rent over a given extent of wire. The phenomena to which contrivances for this end 
have been applied, are such as are observable by the sense of sight alone. ‘The obser- 
vation of the interval is necessarily independent of the observer’s internal mechanism, or 
habit, or personal equation as it is termed; and it is not a matter of primary importance 
to settle, or posit in actual time, the beginning or the end of the phenomenon. 
The other purpose in measuring minute intervals of time, is to determine the fraction 
of a second occurring between two phenomena, one of which is fixed in time, such as the 
beat of a clock, or the contact of its hand with a point, and the other is an incident occu- 
pying no assignable time, such as the contact of a star with a micrometer wire, or with 
the limb of the moon, &c. In this case the action of a single sense, or of more than one, 
may be put in requisition, There are two phenomena, each of which has to be posited in 
time; the interval between them, as to its length, depending on the relation of both to 
absolute time. The observation of the interval may or must be affected by the observer's 
mechanism of sense, or habit, or by his personal equation, in regard to both phenomena. 
The astronomer has-therefore forms of mechanism of three very different orders to attend 
to. There is, first, his own internally; next that of his time-measurers; and lastly, that 
of the universe, which he aims at illustrating. The two former he has to study by com- 
parison with the last, which alone affords him true position and dimension in time; while 
all his conclusions in regard to it, are dependent on his right use of the others. The in- 
VOL. x1.—l5 
