32 PHILOSOPHICAL TRANSACTIONS. [aNNO 1776. 



Others. It is very much to be wished therefore, that some means were used to 

 estabHsh a uniform method of proceeding : and there are none which seem more 

 proper, or more Hkely to be effectual, than that the r. s. should take it into 

 consideration, and recommend that method of proceeding which shall appear to 

 them to be most expedient. 



Of the Barometer, Rain-gage, Wind, and Hygrometer. 

 The barometer is of the cistern kind, and the height of tlie quicksilver is 

 estimated by the top of its convex surface, and not by the edge where it touches 

 the glass, the index being properly adapted for that purpose. This manner of 

 observing appears more accurate than the other; because, if the quicksilver 

 should adhere less to the tube, or be less convex at one time than another, the 

 edge will, in all probability, be more affected by this inequality than the surface. 

 I prefer the cistern to the syphon barometer, because both the trouble of 

 observing and error of observation are less ; as in the latter we are liable to an 

 error in observing both legs. Moreover, the quicksilver can hardly fail of 

 settling truer in the former tlian in the latter ; for the error in the settling of the 

 quicksilver can proceed only from the adhesion of its edge to the sides of the 

 tube ; now the latter is affected by the adhesion in 2 legs, and the former by 

 that in only ] : and besides, as the air has necessarily access to the lower leg of 

 the syphon barometer, the adhesion of the quicksilver in it to the tube will most 

 likely be different, according to the degree of dryness or cleanness of the glass. 

 It is true, as Mr. De Luc observes, that the cistern barometer does not give 

 the true pressure of the atmosphere ; the quicksilver in it being a little depressed 

 on the same principle as in capillary tubes. But this does not appear a sufficient 

 reason for rejecting the use of them. It is better, I think, where so much 

 nicety is required, to determine, by experiment, how much the quicksilver is 

 depressed in tubes of a given bore, and to allow accordingly. By some experi- 

 ments which have been made on this subject by my father. Lord Charles 

 Cavendish, the depression appears to be as in the following table : 



In this barometer, the inside diameter of the tube is about .25 of an inch, 

 and consequently the depression is .05 ; the area of the cistern is near 120 

 times as great as that of the bore of the tube ; so that as the quantity of quick- 

 silver was adjusted when the barometer stood at 294, the error arising from the 

 alteration of the height of the quicksilver in the cistern can scarcely ever amount 



