308 



Vi'dity of the Sympiesometer 



[Oct. 



sudden alarms by sinking tvvo or three divisions on the approach of a 

 comparatively trifling squall. This remark I found perfectly just, for 

 the Sympiesometer, by itself, does certainly give alarming indications 

 on trifling occasions, but while the barometer is observed along with 

 it, the remedy is at hand. The latter takes no notice whatever of 

 sudden squalls, though very violent during their short continuance, and 

 I found in Captain Horsburgli's excellent treatise on the navigation to 

 India, that this was, in his opinion, a most serious objection to the 

 barometer. I could not but be struck vv^ith the beautiful manner in 

 which the Sympiesometer removes this objection, for it just steps in to 

 offer its aid, at the very point where it was wanted. It invariably fore* 

 told the approach of these squalls, indicating their violence by the 

 number of divisions through which it descended, while the barometer 

 stood perfectly, as far as the eye aided by a good microscope could 

 judge, unmoved and heedless. Hence then we concluded, that, if the 

 Sympiesometer fell alone, no danger was to be apprehended, as the 

 squall would soon pass away, and facts invariably supported this con- 

 clusion, for we v\^ere not once deceived ; on the other hand, when the 

 barometer and Sympiesometer fell steadily together, we were sure to 

 have a gale of greater or less violence. In our experience of this we 

 were only once led astray, a very serious depression of the two instru- 

 ments having been for some time observed, but no wind made its ap- 

 pearance. During the whole of the day hov/ever, in which these ob- 

 servations were m.ade, Vve were enveloped in a thick fog bank, which 

 may offer a solution of this apparent contradiction, the elasticity of the 

 air and its consequent power to exert the ordinary pressure being al- 

 ways diminished by the presence of a aqueous vapour. This was on 

 the 5th June, lat. 37^ 10 'S., long. 27 57' E. Bar. 29-70 inches, Symp* 

 28.09 per scale. Ther. 64° 40'. The lowest point to which the 

 barometer fell during the voyage was 29. 25. inches, and the corres- 

 ponding point of the Sympiesometer was also its lowest, 27- 68 per 

 scale, June 14th, lat. 37o 26' long. 61. 27 E. Ther. 62. 10. wind westerly 

 and blowing an exceedingly heav)'" gale. 



The susceptibility of the Sympiesometer enabling it to detect changes 

 unappreciable to our senses, frequently produced a rise in the column 

 while the gale seemed unabated. This was always hailed as a favour- 

 able sign, and we seldom were wrong in concluding that the greatest 

 force of the storm was expended, and that before long the wind would 

 lull. One of the most striking instances of the efficient aid the two 

 instruments would afford when observed together, was just before the 

 storm of the 1 3th of June, A calm and beautiful day was succeeded by 



