SCIENTIFIC SUMMARY. 
213 
The Microrheometer. — Mr. J. B. Hanway gives the above name to an 
instrument for measuring the phenomena of the flow of liquids through 
capillary tubes, which has been termed “transpiration,’’ a word already 
applied to a totally different process occurring in gases, but for which he 
proposes to substitute the term “ microrheosis,” which has the defect of being 
somewhat hazy in its etymology. The apparatus is so arranged that when 
the liquid is introduced the pressure and temperature, as well as the atmo- 
sphere in which the experiment is conducted, may be varied, while the 
thermometer is at the mean point of the system. The determination of the 
relation between the chemical composition of a liquid, as well as its tempera- 
ture and its rate of flow through a capillary tube, was originally worked out 
with great accuracy by Poiseuille as regards pressure and dimensions of tube. 
In examining saline solutions he used percentages instead of equivalent 
values of the body dissolved, and in consequence could make nothing of the 
numbers arrived at. Water runs about five times as quickly at 100° 0. 
as at 0°. Mr. Hanway gives a curve for water from 0° to 100°, the differ- 
ences of rate being smaller as the temperature rises. 
In saline solutions the rate of flow does net depend on any of the 
mechanical features of the salt, such as crystalline form, specific volume, 
solubility, &c., but on the mass of the elements forming the substance, and 
the amount of energy expended in its formation. Each element lia3 a value 
of its own, which is continued in all its compounds. The greater the com- 
bining value of an element the higher its rate. The flow also varies with 
the amount of energy in the compound ; thus nitrates stand highest, then 
chlorides, and lastly sulphates. 
Measurement of Powerful Electric Currents. — Mr. John Trowbridge, of 
Harvard University, contributes a paper on this subject to the American 
Academy of Science. He divides the methods into four — 1, the galvano- 
metric ; 2, the electrometric ; 3, the thermal ; and 4, the electrodynamo- 
metric. No. 1 requires the galvanometer to be shunted by a wire of very 
small resistance. Any error in measuring this small quantity, and any 
heating of the shunt itself, multiply the whole observations by these errors ; 
indeed, a large quantity is measured by operating on a small fractional part, 
the hundredth or the thousandth part of itself. Method No. 2 takes the 
difference of potential of two points in a closed circuit, and is liable to 
leakage and want of constant charge in the electrometer. No. 3 depends on 
the law that the heat developed in a circuit = C 2 Bi, and O can be deduced 
from measuring the rise of temperature in a given volume of water. It is 
liable to errors of conduction, radiation, and of thermometers. 
Method No. 4 employs Weber’s well-known electrodynamometer, con- 
sisting of a movable coil hung from a bifilar suspension which conducts the 
current, between fixed coils on either side of the movable one. But such an 
instrument, as ordinarily constructed, would be quite incompetent to convey 
the large currents to be tested without heating and disorganisation. Shunts, 
as above stated, are to be avoided. 
The instrument actually used is described. The large fixed coils are of 
copper band, thirty-five mm. broad and one mm. thick, in six turns each, 
insulated with vulcanite, and left their own thickness apart, so as to allow a 
free circulation of air. The current was not sent through the bifilar suspen- 
