1838.] 



as a Marine Instrument. 



SOU 



an equally calm and beautiful night, not a cloud was in the sky, and 

 the edges of the full moon were clearly and distinctly defined, 

 with not a trace of any halo or other symptom of unsettled weather ; 

 but the Sympiesometer and barometer has been falling steadily and 

 progressively together, and every one seemed to think that for once they 

 would be proved not infallible. About 9 or 10 p. m., however, the clouds 

 began gradually to gather on the horizon, and the waves in the distance 

 shewed their white crests, the sky became troubled, the moon seemed 

 v/atery-like ; every preparation accordingly was made for a storm, and 

 about midnight the wind rose, and gradually strengthened till the noon 

 of the same day, when it blew a heavy gale. On the night of the 14th 

 we were in imminent danger, and had the instruments not given us time- 

 ly warning few of us might have lived to tell the tale. Few, after this 

 proof of their efficiencyj thought of despising their prognostications. The 

 three rules of Mr. Hemmer, in regard to the circumstances which regu- 

 late the rising and falling of the mercury in the barometer, I found 

 very distinctly indicated by the Sympiesometer. These are : 



1. First when the sun passes the meridian, the barometer, if in the 

 act of falling, continues to do so in an accelerated degree. 



2. When the sun passes the meridian, the barometer, if in the act 

 of rising, falls or becomes stationary, or rises more slowly. 



3. When the sun passes the meridian, the barometer, which is sta- 

 tionary, falls, if it has not risen before or after being stationary ; in 

 which case it usually becomes stationarj^ during the sun's passage. 



These three rules all rest on the single fact, that, about noon, the 

 pressure of the atmosphere becomes diminished ; this result I clearly 

 observed with the sympiesometer, and my remark, made in ignorance 

 of the above rules, was confirmed by the experience of Captain Edmonds, 

 who had observed this fact frequently during the three years he had 

 used the sympiesometer. My observations are by no means sufficiently 

 extensive for me to state definite quantities, I only remarked the de- 

 cided tendency of the column of oil to lower its level about noon, and 

 to rise again in the after part of the day. This is clearly connected 

 with the sun's position in the heavens, perhaps the greater quantity of 

 aqueous vapour present in the atmosphere, and due to the increased 

 evaporation produced by his noon-day heat, may offer a solution of this 

 difficulty, or perhaps it may be owing to some change of the electrical 

 state of the atmosphere, due to the variable temperature of its different 

 parts. There are no means of deciding, that I am aware of, what is the 

 true cause of this, and speculation will not tend to settle the point. 



The prevalence of different winds endues the atmosphere with van- 



