16 On the Nature and Combinations 
would seldom exceed one-tenth of a second; and the errors 
arising from mechanism in this micrometer cannot be of much 
consequence, since the eye-tube is never drawn out further than 
~f;ths of an inch; for 2:6” x 6 ( = 15-6”) is nearly = one divi- 
sion of the micrometer. 
From the comparative micrometer measures taken at the Ob- 
servatory, Armagh, by the Rev. Dr. J. A. Hamilton, it appears 
that the divided object-glass micrometer and the wire-micrometer 
are each liable to an error of 3” in taking the sun’s diameter*. 
On the divided object-glass micrometer, Dr. Hamilton ob- 
serves, ** its imperfections are; that, to different eyes, and under 
different circumstances of the same eye, the length of the focal 
distance, that suits distinct vision, will vary, and of course the 
quantity of the measures given by the scale is liable to a small 
variation. 
‘¢The principal defects of the wire-micrometer are; the dif- 
ficulty of judging accurately of bisections, or contacts of the 
fine wires, by the limbs to be measured; and the impossibility 
of observing any diameter, except one perpendicular to the 
equator.” 
In the construction of my micrometer, I endeavoured to avoid 
all optical illusions, and all mechanism that might produce any 
sensible error. 
1 remain, dear sir, 
With much regard, 
Your very obedient servant, 
Lynn, Dec. 29, 1815. Ez. WALKER. 
V. Onthe Nature and Combinations of a newly-discovered vege- 
table Acid ; with Observations ou the malic Acid, and Sug- 
gestions on the State in which Acids may have previously 
existed in Vegetables. By M. Donovan, Esq. Communt- 
cated by W. H. Wottasron, M.D. Sec. B.S. 
{Concluded from vol. xlvi. p. 444.] 
Observations on the Malic Acid. 
Ix 1785, during an examination of different fruits and berries, 
Scheele discovered that gooseberries, beside lemon acid, con- 
tained one of a peculiar nature: this he afterwards found to 
exist in apples, without, as he thought, a sensible admixture of 
any other. On this account he gave it the name of apple acid, 
or malic acid. 
He also ascertained, that by the action of nitrous acid on sugar, 
* Irish Transactions, vol, x. 
a substance 
