as a Afar hie Instrument. 



311 



4. If a storm seems gathering, of which the barometer takes no no- 

 t.je, but by which the Syrapiesometer alone is affected, a short squall 

 will be the result. No alarm need be entertained, for here the barome- 

 ter completely checks and corrects the Sympiesometer. 



5. A simple change of wind may produce a depression of the Sym- 

 piesometer ; of this the barometer takes no notice. 



6. A slight allowance must be made for the depression at noon. 



The preceding six remarks exhibit the whole of the results of practi- 

 cal importance. There exists an impression, that both the barometer 

 and Syrapiesometer lose their power within the tropics ; unfortunately we 

 had no good opportunity of putting this to a complete test. The Sym- 

 piesometer, however, as far as we could judge, retained its sensibility, as 

 its indications varied, even during the continuance of the fine weather 

 we had. Why the pressure of the atmosphere should cease to act, or ra- 

 ther to exhibit the proofs of its action, within the tropics, it is difficult 

 to conceive ; the indications of the instrument will of course be more 

 vitiated by the increase of temperature, and the larger quantity of mois- 

 ture present in the air, but that the same cause will produce similar ef- 

 fects, either at the Equator or the Pole, is certain, and, if it is the case 

 that barometric indications are not trustworthy within the tropics, we 

 must look for the cause in some concomitant circumstances. I found the 

 minds of those naval men I came in contact with, firmly impressed with 

 this idea, which they said they based on experience, without being able 

 to account for it, yet there are instances on record, in which, on the ap- 

 proach of storms, or hurricanes, immediately under the line, the mercury 

 has fallen upwards of an inch. The deviations of the mercury from its 

 mean annual height are certainly far greater, and more frequent, towards 

 the Poles, than near the Equator, the reasons mentioned above of increas- 

 ed temperature, and diminished elasticity may, to a certain extent, ac- 

 count for this. Our information is however by no means satisfactory; 

 much must yet be done in meteorology before any thing can be offered 

 worthy of serious consideration. The circumstance has been noticed, 

 simply in consequence of its having been suggested, and as a point on 

 which more decided information would be interesting. The many prac- 

 tical rules, based on experience, whereby seamen judge of the weather, 

 and prognosticate coming storms, or cahns, from the appearances of the 

 heavens, are accompanied with considerable interest. The first I no- 

 ticed, was the indications of wind coming from a certain quarter, by the 

 appearance there of Hashes of lightning. To this considerable attention 

 was paid, and it was found generally correct, when the horizon was free 

 from clouds; in the quarter whence the lightning proceeded. If however 



