116 0?i the Longitudinal Eoepansion and Contraction of Wood. [Jan. 



XT t e 



Is ovember 5 

 12 



20 

 26 



December 1 

 6 



No. 1. 



No. 2. 



No. 3. 



No. 4. 



No. 5. 1 No. 6. 



4-0.0070 

 0.0069 

 0.007S 

 0.0069 

 0.0039 

 0.0052 



-f 0,0008 



0.0 

 +0.0005 

 -0.0017 

 0.0047 

 0.0020 



r\ r\r\ r* r\ 



+0.0060 

 0.0036 

 0.0042 

 0.0027 

 0.0009 

 0.0034 



—0.0116 

 0.0102 

 0.0096 

 0.0104 

 1 0.0122 

 1 0.0113 



-f-0.01 07 14-0.0089 

 0.0085 0.0092 

 0.0101 0.0103 

 0.0077 0.0092 

 0.00421 0.0070 

 0.0083 0.0083 



17. — The nionth of November was extremely damp from the copi- 

 ousness of the monsoon, during which the whole of the rods attained 

 their greatest length, and on the 20th of that month, the mahogany rod 

 was longer than before or after, but it seems to have suffered a perma- 

 nent contraction of about one-hundreth part of an inch. As far as 

 these experiments go, it would seem that varnishing wood does not pre- 

 vent its being affected by humidity, and that its variations of length,from 

 the hygrometric state of the atmosphere, are so considerable, as to ren- 

 der it unfit, at least at Madras, for being employed in the construction 

 of a pendulum for a correct time-piece. This is evidently the case with 

 the specimens under notice, but it would not be proper to assert that 

 all kinds of wood, however prepared, would also be unfit, for I have 

 heard it said that a clock with a wooden pendulum has been known in 

 London to keep perfect time. There is one advantage that a clock in 

 England has over clocks in this country; they are not so much subject 

 to the free action of the atmosphere; the free air is shut out in Eng- 

 land by glass windows, while in this country it blows through our 

 houses freely, moist or dry, all the year round. This circumstance 

 makes a great difference, when it is known that only the thousandth 

 part of an inch in the length of a pendulum vibrating seconds, will 

 throw the clock cut of mean time, one second in 24 hours ; and the 

 thousandth part of an inch is but a fine line. 



18. — One very useful hint to the meteorologist may be gathered from 

 these experiments ;— they shew that perfectly well seasoned mahogany, 

 varnished, or not varnished, varies considerably in length according to 

 the state of the atmosphere. Both mountain, and other very good 

 barometers are fitted up in wood, and to the wood, which is generally 

 mahogany, the scale of inches is affixed. Mountain barometers read 

 oflf to the 0.001 (the thousandth part) of an inch, which is a minute 

 quantity ; but it is extremely probable that such minuteness is in prac- 

 tice, not a real but a fictitious degree of accuracy ; for the expansion 

 and contraction of the wood, in drv or wet weather, will amount to 



