VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS 53 



to SO much as -Hroth of an inch. As the tube appeared to be well filled, it was 

 thought unnecessary to have the quicksilver boiled in it ; but that is certainly the 

 surest way of filling a barometer well. 



The principal reason of setting down the mean heat of the thermometer 

 within doors, during each month, in the journal of the weather, is this : sup- 

 pose that any one desires to find the mean height of the barometer in any 

 month, corrected on account of the heat of the quicksilver in the tube ; that 

 is, to find what would have been the mean height, if the quicksilver in the tube 

 had been constantly of a certain^ given heat. To do this, it is suifficient to take 

 the mean height of the barometer, and correct that according to the mean heat 

 of the thermometer ; the result will be exactly the same as if each observation 

 had been corrected separately, and a mean of the corrected observations taken. 

 For example, suppose it is desired to find what would have been the mean 

 height of the barometer in the month of August 1775, if the quicksilver during 

 that time had been always at 50 degrees of heat : the mean of the observed 

 heights is 29.86 inches, and the mean heat of the thermometer is 6d" or 

 50 + 15. The alteration of the height of the barometer by 15° of heat, ac- 

 cording to M. De Luc's rule, is .047 inches ; consequently the corrected mean 

 height is IQ.SIS. 



The vessel which receives the rain is a conical funnel, strengthened at the top 

 by a brass ring, 12 inches in diameter. The sides of the funnel and inner lip 

 of the brass ring are inclined to the horizon, in an angle of above 65°; and the 

 outer lip in an angle of above 50° ; which are such degrees of steepness, that 

 there seems no probability either that any rain which falls within the funnel, or 

 on the inner lip of the ring, should dash out, or that any which falls on the 

 outer lip should dash into the funnel. This vessel is placed on some flat leads 

 on the top of the Society's house. It can hardly be screened from any rain by 

 the chimnies, as none of them are elevated above it in an angle of more than 

 25° ; and as it is raised 3-i. feet above the roof, there seems no danger of any 

 rain dashing into it by rebounding from the lead. 



The strength of the wind is divided in the journal into 3 degrees ; namely, 

 gentle, brisk, and violent or stormy, which are distinguished by the figures 

 ], 2, and 3. When there is no sensible wind, it is distinguished by a cypher. 



In the future journals of the weather will be given observations of the 

 hygrometer. The instrument intended to be used is of Mr. Smeaton's construc- 

 tion, and is described in Phil. Trans, vol.61, p. IQS. It is kept in a wooden 

 case, made so as to exclude the rain, but to leave a free passage for the wind, 

 and placed in the open air, where the sun scarcely ever shines on it. The instru- 

 ment and case are both a present to the Society from Mr. Smeaton. The 



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