﻿74 Royal Society. 



These results, when compared with the previous determinations for 

 these same instruments, would seem to show that a somewhat better 

 result is obtained when the exhaustion is carried on more slowly, and 

 hence that the corrections depend, to a considerable extent, on the 

 nature of the treatment received. No. 8 seems to be more constant 

 under different treatment than No. 9. 



From all these experiments we may perhaps conclude as follows : — 



(1) A good aneroid of large size may be corrected for temperature, 

 by an optician, so that the residual correction shall be very small. 



(2 a) If an aneroid correct to commence with be used for a bal- 

 loon- or mountain-ascent, it will be tolerably correct for a decrease of 

 about 6 inches of pressure. 



(2 /3) A large aneroid is more likely to be correct than a small one. 



(2 y) The range of correctness of an instrument used for mountain- 

 ascents may be increased by a previous verification, a table of cor- 

 rections being thus obtained. 



(3 oc) If an aneroid have remained some time at the top of a moun- 

 tain, and be supposed correct to start with, then it will give good 

 results for about 8 inches of increase of pressure. 



(3/3) A large aneroid is more likely to be correct than a small one. 



(3y) If the aneroid has been previously verified, it is likely to give 

 a better result. 



(4) After being subjected to sudden changes of pressure, the zero 

 of an aneroid gradually changes ; so that under such circumstances it 

 ought only to be used as a differential and not as an absolute instru- 

 ment — that is to say, used to determine the distance ascended, making 

 it correct to begin with, or to ascertain the distance descended, ma- 

 king it correct to begin with — it being understood that the instrument 

 ought to be quiescent for some time before the change of pressure is 

 made. 



Before concluding I ought to mention that most of the experi- 

 ments herein described were undertaken and executed in a very careful 

 manner by Mr. T. "W. Baker. 



" On an Easy Method of measuring approximately the Intensity 

 of Total Daylight." By Roger J. "Wright, Esq. 



An easy method by which the amount of light may be at any time 

 measured and registered appears to be still wanting. I would sug- 

 gest the following plan, by which I believe the desired object may 

 be attained. 



A B is a rod of solid metal, terminated by a heavy base, which 

 keeps the rod in a perpendicular position. C D is a hollow tube, 

 blackened inside, of such a diameter as exactly to fit and slide over 

 A B. The extremity, B, of the rod A B is painted of a snowy white, 

 with a jet-black spot in the centre, as shown in the figure. On AB 

 is marked the scale, beginning with zero at A. The tube is pushed 

 over the rod till the extremity C coincides with the zero line at A. 



The method of using this instrument is as follows : — Draw the 

 tube gently up the rod, at the same time looking steadily at the 

 black spot before mentioned. It will be found, as the tube ascends, 



