270 
Strictly speaking, measure includes weight, which is the 
measure of the gravitation of bodies towards the centre 
of gravity. And measures of capacity also are almost uni- 
versally derived, not from their cubical dimensions, but 
from the weight of pure water contained in them under 
determinate conditions as to temperature and atmospheric 
pressure, 
The measure of temperature is based upon the observed 
rate of linear expansion by heat of a body selected 
for this purpose, generally mercury, taking as constant 
units the temperature of melting snow or ice, and of 
water boiling under determinate atmospheric pressure. 
In defining measure, it should be added that itis also 
applied to the measure, or (as it is termed) admeasure-- 
ment of the tonnage of ships, being a determination of 
the weight a ship is capable of carrying, with relation to 
its measure of cubic capacity ; to value in relation to 
a monetary unit ; to timeand duration in relation to the 
unit of a mean solar day or a second, its 86,400th part ; 
to velocity, by combining the measure of extension with 
that of time or duration ; to mechanical work, the unit of 
which is a horse power, as it is commonly termed, or more 
properly the power of raising 33,000 lbs. one foot in 
one minute, thus combining the measures of linear 
extension, weight, and time ; to angles, the unit being a 
degree or the 360th part of a circle described from the 
point of junction of the two straight diverging lines 
forming the angle ; &&. &c. It is not, however, proposed 
here to refer further to these measures or to the scientific 
questions connected with them. 
The measure of volume, or bulk of a body, as compared 
with that of another body differing in volume but equal 
in weight, is shown by its density, and is also expressed 
in terms of a fixed standard unit. The densities 
of bodies are in the direct ratios of their masses, or 
quantity of matter, and in the inverse ratios of their 
volume. 
The density of a body is defined to be the mass con- 
tained in a unit of volume, when referred to a uniform 
standard. The specific density is to be distinguished from 
its specific gravity, which shows its weight in relation to 
its volume, also when referred to a uniform standard. 
The specific gravity of a body is defined to be the 
weight of a unit of its volume. 
The specific gravity of a body is the quotient of its 
density when divided by the density of that substance 
which is considered as unity. Pure water is generally 
adopted as such unity. But since both these densities 
vary with the temperature—because the same invariable 
quantity of matter which the body contains is always 
distributed over its whole volume, and this is variable with 
the temperature ; so that, generally speaking (with some 
exceptions, pure water, for instance, at certain tempe- 
ratures), the body, at a higher temperature, has less den- 
NATURE 
a 
| from the programme, the meeting 
sity than at a lower temperarure—we must fix a certain | 
temperature at which the body, as well 
be considered. It is not necessary that this fixed tem- 
perature should be the same for the body and the water, 
its choice for both being quite arbitrary. 
For bodies the most convenient standard temperature 
for expressing their density seems to be that of one of the 
fixed points of the thermometer; and the temperature of 
melting ice or snow (32° F. or 0° C.) is generally 
adopted. For pure water, there is a maximum of density 
which occurs at nearly 39° F, or 4° C., and this maximum 
density of pure water is generally adopted as the unit of 
density. 
_ The sign A prefixed to the symbol of any weight, with 
its numerical value following, denotes the ratio of the 
density of the weight at the temperature of melting snow 
to the maximum density of pure water. 
The relation of the bulk or volume of a body to its 
weight is expressed both by its density and its specific 
gravity, these terms being often used indiscriminately. 
as the water, must | 
AS GR ae) Coy ae 
“ St 
| Fuly 31, 1873, 
But the former term is more strictly applicable to solid 
bodies, and the latter to liquids and gases. 
To ascertain the density of a body, it is requisite that 
its volume should be determined, as the density cannot 
be directly found. The actual volume may be deter- 
mined— 
1. Either by cubic measurement, when the form of the 
body admits of this measurement being actually made ; 
but this occurs but rarely. 
2. Or by ascertaining its specific gravity, from de- 
termining the difference of its weight when weighed in 
air and in water. This is the readiest and most accurate 
mode of determining both its volume and its density, but 
the immersion of a body in water is not always prac- 
ticable, or it may be injurious to the body under expe- 
riment. H, W. CHISHOLM 
(To be continued.) 
NOTES 
Art the Meeting of the Paris Academy of Sciences, M. Fer- 
dinand de Lesseps was elected an “* Academicien libre” in the 
place of M. de Verneuil, deceased. M. de Lesseps obtained 33 
votes ; M. Breguet, 24; MM. du Moncel, Jacquemin, and Sedillot 
teach. M. de Lesseps thus obtained 2 votes beyond the abso- 
lute majority required to render an election valid, and was 
therefore declared elected. The number voting, 60, was large. 
THE forty-first Annual Meeting of the British Medical Asso- 
ciation will be held in King’s College, London, on Tuesday, 
Wednesday, Thursday, and Friday, August 5th, 6th, 7th, and 
8th. The President-elect is Sir William Fergusson, Bart., 
F.R.S. The following are the six sections into which the meet- 
ing will be divided, and in each section a very large number of — 
papers is already entered to be read :—Section A, Medicine; 
B, Surgery ; C, Obstetric Medicine; D, Public Medicine; E, 
Psychology ; F, Physiology. The sections will meet in rooms 
of the College appropriated for the purpose, and the Annual 
Museum of objects of interest in connection with medicine, sur- 
gery, and their allied sciences will be arranged in the Library of 
the College. The President’s address will be delivered at 3 P.M. 
on August 5, and in the evening the Lord Mayor will hold a re- 
ception at the Mansion House. The following public addresses 
will be delivered :—On August 6, an address on Medicine, by Prof. 
E. A. Parkes, M.D., F.R.S. ; on August 7, an address on Surgery, 
by Prof. John Wood, F.R.S. ; and on August 8, an address on ; 
Physiology, by Prof. Burdon Sanderson, F.R.S. The President 
and Council of the Royal College of Surgeons hold a reception — 
on the evening of August 6, and several excursions have been 
arranged to take place during the meeting. Altogether, to judge 
promises to be a very success- 
ful one. 
Tue Royal Archeological Institute commenced its annual 
session at Exeter, on Tuesday, when the Mayor and Cor- - 
poration held a reception at noon. The President, the Earl of 
Devon, thereafter delivered his inaugural address on the adyan- 
tages of the study of Archeology, and in the afternoon an 
excursion took place to Rougemont Castle. In the evening, 
again, the Mayor held a reception in the Albert Museum. The 
Sectional Meetings commenced yesterday, and several interesting 
excursions have been arranged. The Sections are, Antiquities, 
Architecture, and History. One of the most attractive accoms 
paniments of the Exeter meeting is the formation of a temporary 
Museum and Portrait Gallery, 
THE French Association for the Advancement of Science 
commences its second session at Lyons on August 21, the con 
cluding meeting to be held on August 28. As was the case a 
Bordeaux, there will be General Meetings, Meetings of Sec- 
tions or Groups, Scientific Excursions, and Public Lectures. A 
