[83] fisheries of the united states. 727 



Chronometer. 



Used for ascertaining longitude. "This instrument [the chronom- 

 eter] in its most perfect form has resulted from the demands 

 of navigation, and upon its performance the safety of commerce 

 in a large measure depends. As an instrument of precision it 

 is entitled to the highest rank, and especially when it is con- 

 sidered that, unlike in the case of an astronomical clock, it is 

 not almost daily compared with actual time observations to> 

 determine its error and rate, but on the contrary it is to be 

 depended upon for weeks and even months, and the time ob- 

 servations are solely to find the local correction, and hence the 

 longitude at sea. The method of construction has long been 

 well established, and the only differences usually to be found 

 in the work of different makers, beyond minor differences in 

 the arrangement and size of the parts, are in respect to the 

 adjustment for temperature. * * * It is usual for Amer- 

 ican manufacturers to import the ebauche from England, and 

 the work performed by them consists in finishing up the parts 

 of the train, and in making the adjustments for isochronism 

 and temperature, upon which the time-keeping properties de- 

 pend. Messrs. John Bliss & Co., however, exhibited chro- 

 nometers which had been wholly constructed by them, and 

 hence of strictly American manufacture throughout." (Extracts 

 from Beport of Prof. James C. Watson to the United States 

 Centennial Commission, 1876.) John Bliss & Co., New York. 



Parallel rules. 



Two sizes of Sigsbee's patent parallel rules. John Bliss & Co., 

 New York. Used for shaping courses on charts. 



" The principal advantages of this rule are as follows : (1.) Perfect 

 ease of movement, due to the method of hinging the blades. 

 (2.) The blades may be raised over thumb-tacks, creases or 

 torn edges of charts ; and are self-lifting when moved over the 

 surfaces of uneven tables, reducing the probability of slipping. 

 (3.) In projecting a course, if it be desired to examine for 

 soundings or shoals that part of the chart covered by the rule, 

 the movable blade may be thrown back for that purpose with- 

 out shifting the other blade ; or it may be stood on edge along 

 the projected course, rendering pencil lines unnecessary. (4.) 

 A slight pressure with the thumb and finger on the two rubber 

 cushions of the fixed blade will prevent the rule from slipping. 

 (5.) A parallel to the edge of a drawing board or block may 

 be drawn by placing the blades at right angles to each other; 

 one blade flat on the paper and the other, flat against the edge 

 of the board or block. (6.) The entire length of the inner 

 edges of the blades may be used for ruling, without interference 



