214 EEPOET OF COMMISSIONER OF FISH AND FISHERIES. [98] 



The ship's head, while building, was toward north, 29° 30' W. ; the 

 ways were in latitude 39© 44^ K, longitude 75°' 33' W. 



Of the chronometers on board, the Bliss & Creighton was loaned by 

 the Kavy Department, the Parkinson & Frodsham is the property of 

 the commanding officer, and the Negus pocket chronometer, of the 

 navigator. The box chronometers were j)laced under the luunge in the 

 chart-room, the transporting cases being screwed to a false bottom on 

 the deck. In this position they are secure from shocks, and the top of 

 the lounge, opening and shutting on hinges, fits tightly enough to pre- 

 vent any great changes of temperature. The lowest mean temperature 

 between ratings during the year has been 57.4° F., and the highest 76° 

 F. On deck the extremes havebeen 14° and 96°. The interval between 

 ratings has usually been ten to twenty-five days. 



The most powerful disturbing element on the rates of the chronome- 

 ters has been the vibration of the hull caused by the dynamo engine, 

 which is usually in operation from dark until 11 p. m. They appear to 

 run equally well together while this vibration takes place every day, 

 and during any material interval that it does not take place at all, but 

 an interruption of either state of repose or vibration is almost invariably 

 accompanied by a change in the second differences in the daily compari- 

 son book, showing that their rates are temporarily disturbed. 



The dates of last cleaning of the chronometers are as follows : Bliss 

 & Creighton, No. 1078, June, 1883 ; Parkinson & Frodsham, No. 1541, 

 March, 1883; Negus, No. 1673, March, 1883; Negus, No. 1674, March, 

 1§83 ; Negus, No. 1696, March, 1883 ; Negus, No. 3702 (pocket), Octo- 

 ber, 1880. 



Ordinarily the Bliss & Creighton has been used for everyday work, 

 the mean of all being taken when greater accuracy was required. On 

 reaching port the chronometers are rated as soon as possible, and the 

 errors corrected back if the discrepancy is greater than the i)robable 

 limits of j)ersonal and instrumental errors of observation and plotting..^ 



The Eogers i)ortable micrometer telescope, loaned by the Bureau of 

 Navigation, Navy Department, is a very reliable and a very useful in- 

 strument. A description and method of using it is to be found in the 

 revised edition of Bowditch's Navigator, page 177. A modification of 

 that method has been adopted in this vessel, by which less computa- 

 tion is required and the necessity is avoided of picking out each time 

 the log. cotangent of such a small angle. It is as follows : 



The greatest angle this instrument can measure is 1,750 micrometer 

 divisions, or about 1° 45', and it is seldom that an angle of over one- 

 half or three-quarters of that is observed with it. In such small angles 

 the functions may be considered as proi)ortional to the arcs ; that is, 

 the cotangent of the angle measured is equal to the cotangent of one 

 micrometer division divided by the number of those divisions. The log. 

 cotangent of one division being accurately determined once for all, the] 

 rule for finding the distance is simply to add that function to the loga 



