S. P. Langley—Allegheny System of Time Signals. 877 
in experiments on binocular vision, etc., the results are not to 
be relied on as having been obtained with an instantaneous 
illumination, as is abundantly shown by the large total dura- 
fons so often met with in this investigation. 
Finally, by the use of large rotating discs, these multiple 
discharges can readily be exhibited to moderate sized audiences. 
New York, June 29th, 1872. 
Art. XLIV.—On the Allegheny System of Electric Time Sig- 
nals; by Prof. S. P. LANGLEY. 
THE necessity of a uniform standard of time for the railways 
of the United States is one which is growing into importance 
with the increasing extent of our railway system; and we are, 
ere long, in this country, to be called on to settle for ourselves 
4 practical problem which has already been solved in England, 
and which is beginning to make its demand for solution upon 
the managers of our railroads. 
Since past experience shows that their probable adoption of 
4 new and common standard will introduce it to public notice 
and discussion, and then to adoption by cities and individuals, 
it is desirable that this should not be done without the 
direction which intelligent scientific coGperation will give to a 
movement originated by the demands of intercontinental 
c. As few are aware how generally this codperation has 
y been invoked, nor to what extent the public is indebted 
to observatories for increased security of transit, it has seem 
tan account of what has been done in this direction in any 
ne of them would be of interest. 
The earliest introduction of the system of electric automatic 
smission of time-signals, on an extended scale, appears to 
be due to the observatory of Greenwich. 
‘he Astronomer Royal, with Mr. C. V. Walker, commenced 
their use in 1852, carrying for that purpose special wires on the 
les of the South Eastern Railway from Greenwich to London 
Bridge. The subsequent extension of the use of Greenwich 
time under this system has been almost universal throughout 
the United Kingdom, the observatories of Glasgow and 
Liverpool, under the direction respectively of Professor Grant 
and Mr, Hartnup, as well as that of Edinburgh, having taken 
Part in bringing it to its present condition of utility. For an 
instructive and very full description of the methods employed 
at Greenwich, reference may be made to an article m_ the 
perological Journal for April, 1865, by W. Ellis, Esq., 
PRA , to whom, as to all the gentlemen named, the writer 
Am. Jour. Sct.—Turrp Serres, Vou. IV, No. 23.—Nov., 1872. 
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