214 Application of the Galvanic Circuit 



In his catalogues of right ascensions of stars the use of the 

 printing method would only introduce a fivefold instead of a ten- 

 fold gain. 



I venture to suggest that our National Expedition to Chili, under 

 Lieutenant Gilliss, will by the use of the printing method, furnish 

 nearly ten times the precision in the value of the parallax of Mars, 

 from his relative transits with the same star, both east arid west 

 of the meridian, that would result from the ordinary method of 

 observation. 



There is, however, no one branch of practical astronomy, or 

 coast survey operations, where the benefit of the automatic clock 

 register will be so great as in the determinations of relative lon- 

 gitude of the stations. 



One night's work with the printing method will be worth as 

 much as any single campaign has been heretofore between any 

 two stations. 



The importance of this branch of the duties of the coast sur- 



vey induces me to dwell more at length on the subject. Your 



operations propose, for the first time in the history of geodesy, to 

 measure astronomically a long arc of a parallel with a precision 

 commensurate with the former and present astronomical measures 

 of arcs of the meridian. 



I would, for this purpose, respectfully recommend the mount- 

 ing of one of Mr. Wheatstone's, or of Dr. Locke's, clocks at 

 some central astronomical station, having easy telegraph commu- 

 nication with the northern and southern points of the coast sur- 

 vey. This central station, and any other one or more stations 

 intended to be compared together, may be supplied with transit 

 instruments, constructed with special reference to the advantages 

 of the printing method. I would respectfully recommend the 

 use of a transit instrument with a frame of a cast iron box, and 

 with three or more doubled faced levels on each side suited to 

 both zenith and nadir pointings. When not too cumbersome, 

 as in the case of the coast survey instruments, I would wish for 

 an azimuth adjustment, with graduated micrometer head, like 

 the Simms transit instruments now in use in the coast survey. 

 I would wish to retain the principle of rapid reversals, and would 

 extend it to zenith stars. The inner portion of the pivots may 

 rest on counterpoises with friction rollers. I would wish the re- 

 versing apparatus to have the following advantages, leaving to 

 the mechanician the decision as to the mode of securing them. 

 The time occupied in the reversal and in repointing the instru- 

 ment, by making it revolve on its axis through the double oC its 

 zenith distance before reversal, should not exceed twenty seconds. 

 The pointing may be done by means of an adjustable clamp, 

 suited to the declination of the star. The pivots need not be 

 elevated more than half an inch in order to move free of the 



