Grupp—TZime Signals. 39 
I here append details of the construction of the three parts, A, 
B, and C:— 
(A).—Should be the best procurable timekeeper, preferably 
with a gravity escapement, and furnished with an arrangement 
which would make a delicate electrical contact once in every 
15 seconds. 
Such a clock, mounted in the ordinary way, should give an 
excellent result; but still further to ensure accurate working, I 
would propose that this clock be enclosed in an air-tight case after 
the manner adopted by Professor Becker, of Glasgow, by which the 
pressure is kept constant, and this case placed in another, duly pro- 
tected by non-conducting material, and the whole mounted within 
its double chamber in the basement of the Institution, so that it 
would be practically free from both barometric and thermometric 
changes. 
Such a clock should have very small e77o0s, but no clock is 
without some small safe. The rate+or—of a clock must not be 
confounded with its errors. The very best clock, and that with 
the smallest error, may have a fairly large rate; that is to say, it 
may give 86,401, or 86,399 secs. in the 24 hours instead of 86,400, 
but may preserve that rate steadily, day by day, without any sen- 
sible error from that rate. This is not serious for one day, but if 
allowed to accumulate for a week or a month, it is; and it must 
be remembered that a clock like this standard clock A, being sealed 
up in a case, cannot be readily got at for re-setting; therefore it is 
that I propose to depend on this clock, accurate though it may be, 
for only 24 hours at a time, and have made arrangements in the 
part B to have this—the distributing clock—checked and auto- 
matically corrected by a Greenwich signal, an automatic registra- 
tion being made each day of the extent of the correction of the 
standard clock A. 
Another trouble with sealed-up clocks has sometimes been 
experienced—I refer to the gradual oxidation after a lapse of time 
of the electrical contacts. ‘lhis will probably not be serious with 
the very small current necessary; but if desired to ensure against 
failure in this respect, I would recommend that the air-tight case 
of the standard clock be enclosed in another larger one, forming a 
jacket, both the clock case and its jacket being filled with nitrogen 
