640 



REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



[36] 



merits, although seldom equaling half a degree, tend to cast a doubt 

 upon readings to small fractions; and it may be that the quantity of 

 mercury contained in the small safety bulb at the top of the tube is 

 sufficienl to cause a fractional error when the temperature of the water 

 differs from that of the air at the time of reading. 



"For these reasons, and considering the fact that at depths greater 

 than 1 mile the general ocean temperature falls very gradually if at 

 all, and that observations at far greater depths do not agree in report- 

 ing corresponding differences in temperature, I requested authority 

 from the Commissioner to order an experimental half-dozen of longer 

 tubes of uniform range, and pointed to one-fifth degree Fahrenheit 

 (Plate IV). The specification was as follows: 'The special thermom- 

 eters are required to be of sufficient length to be legibly pointed in 

 fifths of a Fahrenheit degree, and it is particularly desired that all of 

 the instruments now or hereafter ordered shall conform as nearly as 

 possible to the range from 20° to 90° Fahrenheit, as specified in un- 

 let ter of August 6, 1SS6.' (Order dated September G, 1SSG.) 



"As far as can be determined by laboratory experiments the new ther- 

 mometers fulfill all of the desired conditions, and are besides unusually 

 free from index error. It is possible that before the Albatross sails I 

 shall be able to furnish a correction for the small error arising from the 

 expansion of the mercury contained in the smaM safety bulb at the top 

 of the tube after oversetting." 



THE TANNER IMPROVED THERMOMETER-CASE WITH THE SIGSBEE 

 CLAMP AND THE NEGRETTI-ZAMBRA SPECIAL DEEP SEA THER- 

 MOMETER. 



[Plate IV.] 



Fig. 1 shows the apparatus complete, and Fig. 2 a vertical sectional 

 elevation of the case containing the thermometer. 



The thermometer-case is made of brass except the Sigsbee clamp, q, and 

 spiral springs, h. which are phosphor bronze. The frame is cast and 

 the case in which the thermometer is inclosed is an ordinary tube of 

 commercial pattern. 



