296 



THE DEPTHS OF THE SEA. 



[CHAP. vii. 



This experiment at once proved the advantage of 

 the encased bulb. It was repeated with other ther 

 mometers with the same pressure and for the same 

 period of time, and it was found that while the mean 

 difference of the encased bulbs was only 0*95, that of 

 the ordinary deep-sea thermometers was, as in No. 57, 

 7*25. It follows, also, from these experiments, that 

 very nearly all the difference or error is due to pres 

 sure on the full bulb, and that by encasing that bulb 

 we have a nearly perfect instrument. 



The next series of experiments was made to esta 

 blish a scale by which observations by the ordinary 

 instruments might be approximately corrected for 

 pressure. The following table gives the errors of 

 six thermometers at different pressures. The 

 ' standard ' is an encased Miller-Casella, the last 

 a registering minimum thermometer by Casella 

 enclosed in a hermetically sealed glass tube on Sir 

 William Thomson's plan. 



The mean difference for each 250 fathoms in each 

 thermometer is as follows : 



