UNITED STATES ENGINEER OFFICE,

GEOGRAPHICAL AND GEOLOGICAL EXPLORATIONS AND SURVEYS

WEST OF THE IOOTH MERIDIAN, May 1, 1873.

Memorandum of instructions for conducting observations for longitude and latitude at a main or primary astronomical station for the field-season of 1873.

The fixed observatory, with which connection will be made, is in Temple Square, Salt Lake City, Utah, and in charge of Assistant John H. Clark. Every observer is responsible for the receiving of the signals from the connecting-stations for certain reductions of results, as full as possible, and will follow such other instructions that may, from time to time, be given. This station will be changed during the season, probably in the month of August, to the United States engineer observatory, to be constructed at Ogden, Utah.

1.-Observations for Longitude.

In carrying on the astronomical campaign, preference will be given to the longitude-results for those nights that are clear at both stations; that is, the observers in charge, after having completed the necessary arrangements for the observations and exchange of signals, will, from this time on, exchange signals each night that shall be fair at the two stations, unless unavoidable difficulties arise to prevent.

It will be carefully observed that weather-signals are sent by telegraph each evening about 6 p. m. while the exchanges are going on and until the full series at a station shall have been completed. In case of uncertainty as to the possibility for observing for any one night, this may be indicated in the first dispatch; and subsequent dispatch or dispatches, between this time and 10 p. m., shall determine whether exchanges shall be made that night or not.

Some convenient and concise form will be adopted for the weather-signals, and in no case must there be a failure as regards forwarding them.

It may be admissible, contingent upon the stage of the prosecution of the work, should the weather at Salt Lake at about 6 p. m. look unfavorable, that the observer at the distant station should at once conclude to take that night for latitude-work, which may be done after informing the observer at the Salt Lake or the receiving station.

For the conducting of the observations for the exchanges for a single night, the following instructions will be adhered to: The transit of stars for time-determinations, consisting of not less than three time and two circumpolar stars in each position of the instrument, both before and after the transmission of the signals, will be taken. Of course, should the night be sufficiently clear, so that it seems possible to make the time-determinations at both stations, by the modification of the above, so that at one or both of the stations the transits of stars may be made entirely before or entirely after the sending of the signals, the strict following-out of the above is not necessary.

These instructions are made with the understanding that a twenty-six or thirty

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two inch Würdemann instrument is used, with recording-apparatus in shape of a chronograph or register.

The time set for the transmission of signals should be as near 9.30 p. m. local time of the Salt Lake meridian as possible; and great care should be taken that both observers shall be on hand simultaneously, so that as little delay as possible shall ensue in the use of the telegraph-wires.

In the transmission of signals the record is to be made upon the chronograph or register at the two places over a space of five minutes in time, the connecting or Salt Lake station sending for the first five minutes and the distant station receiving, and vice versa.

In addition to these, which may be known as the chronograph-signals, arbitrary signals will be sent, by the use of a break-circuit key, at about ten seconds apart, at fractional parts of a second, making a series of thirty-one arbitrary signals during the five minutes.

In order that the observer at a station may conclude that he has accomplished six nights of first-class observations, it becomes necessary that there should be an exchange of the approximate results. Each observer will therefore send to the other, upon each subsequent night or as soon thereafter as practicable, the approximate error of his chronometer and the mean of seven arbitrary signals, sent and received, selected from the middle of the set of thirty-one. This may be concisely expressed in a telegram.

For the full satisfaction of an observer at the distant station, such further computation shall be sent and received as shall seem necessary to a clear understanding of the case; as an observer will be held responsible should he leave the station and go to another before he is certain that the results upon final computation would prove satisfactory.

2.-Observations for Latitude.

These will be conducted through five complete and clear nights, so that there shall not be less than 175 pairs of observations upon 35 separate and distinct pairs of stars, each pair of which observations shall give a first-class result.

These instructions are furnished to the observers with a view to their clearly understanding the class of results intended, and will always be carried out, unless unforeseen difficulties arise, in which event, as the observers will not hereafter be within speedy communication of these headquarters, it will be necessary for them to adopt immaterial modifications upon their own responsibility. It is, of course, understood that each observer is responsible for and receives the credit of his own work.

A report will be made by each observer, at intervals not exceeding fifteen days, of the work under his charge.

A full daily journal will be kept by each observer. Great care will be taken as to the character of the record upon this journal, which should be clear and explicit. The position of the station in reference to surrounding natural objects should be clearly described, and imperishable meridian-marks firmly planted. When possible, a special survey and plat will be made.

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The day following an exchange of the chronographic record of time and exchangesignals, said signals will be carefully copied into a record-book, which, like all other records, must be made in duplicate.

It is intended that computations in the field shall be carried on to such an extent as circumstances may permit.

The time that can be allotted to each of these stations, in order to accomplish the expected results for the season, is from twenty to twenty-five nights; and as observations are required only for eleven nights, the remaining interval should be employed in computation.

The order of sequence for an astronomical report is furnished herewith, and the records and journal will be so kept as to comprise all the data necessary for the full expression of results in accordance therewith.

Official :

GEO. M. WHEELER,

First Lieutenant Corps of Engineers, in charge.