aes ee 
Annual Report on the U. 8. Coast Survey. 315 
opposite sign, the signal being apparently received earlier than it 
was given. From this, the nullity of both these corrections, 3 
and 4, is fairly inferred, and the interesting consequences follow, 
that ‘the telegraphic method of comparing clocks, distant two 
hundred miles from each other, is free from error when the meth- 
od of coincidence of beats is employed, and that the probable er- 
tor of the longitude, from this method, is the same as the mean 
result of the computed relative correction of the clocks for the 
nights of observation.’ An investigation of the probable value 
of such error shows that, under favorable astronomical circum- 
stances, and with due care in the use of the transit instrument, 
‘the astronomical difference of longitude, between any two sta- 
ions of a trigonometrical survey, may be determined by tele- 
graphic signals, with a degree of precision of the same order as 
that of difference of latitude,’ the inaccuracy depending upon the 
same causes as the deviation of the plumb-line.” 
Magnetic Observations and Instruments.—Observations for 
determining the variation, the dip and intensity of magnetism, are 
Made at the more prominent points of the coast survey, and the 
iNstruments used for variation and intensity, are those invented 
by Weber and Lloyd and modified by Lieut. Riddell. They 
consist of a portable declinometer, with its theodolite apparatus 
for deflections and vibrations. "This apparatus was here first used 
under the immediate direction of the su perintendent, and also at 
humerous detached stations by two assistants, Capt. I. J. Lee and 
Mr. R. H. Fauntleroy, and by two of their aids, and has been 
approved in every case by these gentlemen as convenient, and 
Whenever the azimuth lines of the survey could be used, as 
Siving accurate results. It was also used by Prof. Renwick and 
Dr, Locke, but whether approved or not we do not know. We 
Mention this particularly, because we have learned from a dis- 
tinguished observer abroad, that there is a disposition in England 
to return to the use of the older methods, which is certainly not 
warranted by the experience of the observers of the coast survey. 
he magnetic observations near the city of Baltimore indicate 
that this region is one of considerable local attraction. la 
Maps of the Coast.—It is stated in the Address of the Presi- 
dent of the Astronomical Society of London, that a specimen of 
the new map of England indicates that it has advanced as much 
Upon its predecessors, as its predecessors had upon all other maps. 
The qualities which indicate this advance, we learn from the 
Same Address, are: 1st. That the map is upon the scale Of sty 5; 
or six inches to the mile. 2d. That its details include every 
thing of topographical value which can be placed upon a map 
embracing the traces of the lines of equal heights. 3d. The 
latitudes and longitudes are graduated not to degrees, as on com- 
Mon maps, nor to minutes, as on the largest maps on which we 
Scorn Szrizs, Vol. V, No. 15.—May, 1848. 4} 
