In the sector of a circle, the angle at the centre is a Arc function of the arc and the radius; viz: Angle œ Radius

but this is no equation, except we make an arbitrary choice of units.

Does not almost the whole of dynamics consist of equations (or to speak more properly analogies) between heterogeneous quantitities? But I imagine the falsehood of this kind of reasoning is already sufficiently proved.

I am, Sir,

Your most obedient servant,
THOMAS KNIGHT.

Papcastle, Nov. 3d, 1810.

VI.

Description of an Hygrometer for Gasses, and the Method of
Using it, to subject different Substances to their Action: by
Mr. GUYTON-MORVEAU*.

gasses im

SINCE philosophers have endeavoured to investigate the Ascertaining properties of aeriform fluids, various kinds of apparatus the dryness of have been invented, for placing them in contact with sub- portant. stances, that by their action on them might elucidate their nature, or might form with them new combinations. Of late they have particularly felt the necessity of reducing gasses to the greatest dryness, that the phenomena produced by their essential constituent parts might not be confounded with those, that might result from the decomposition of the water given out by them.

The instrument, which I now lay before the class, appears The instruto me well adapted to both these objects. It was not till ment repeat. I had several times tried it, that I resolved to have one edly tried. finished with great care, and to communicate a description

Annal. de Chim. vol. LXVIII, p. 5. The instrument here described was laid before the physical and mathematical class of the Institute on the 8th of August, 1808.

of

Description of it.

Method of using it.

Explanation of the plate.

of it to those, who are aware that in these delicate experi ments we have never too many means of securing ourselves against the errours of manipulation.

This apparatus being intended to be introduced, very accurately closed, under a receiver the mouth of which is immersed in mercury, it must be mounted wholly in iron. When the vessel is carried above the level of the mercury, it is easy to open it, to give an opportunity for the mutual action of the substance it contains and the gas under the receiver; which is effected by moving the dependent part of the counterpoise, previously disengaging the catch. To keep it in this position, nothing is required but a simple wooden bracket, with a notch cut in it to receive the handle of the instrument, in which it is fixed by a wedge. After having allowed it to stay as long as is necessary for the action required to take place, the glass vessel may be closed by its glass stopple, and taken out of the trough, without danger of any thing escaping from it, or of a single particle of mèrcury getting in, by means of the same catch, which is placed in the middle of the counterpoise, and strongly presses down the cover.

To examine the hygrometrical state of any gas, take the glass bottle out of its collar, weigh it accurately, and fill it with dry pulverized muriate of lime, that has been in fusion; which likewise must be weighed. Having replaced it, and shut down the cover close, introduce it under the receiver, and then raise the cover. The weight gained by the muriate of lime will indicate the quantity of water absorbed. Explanation of the figures..

Pl. VIII, fig. 1, represents the apparatus introduced under the receiver, the glass stopple raised by the hinged cover, to which it is cemented. The cover is kept up by the catch g; which in this situation is placed under the cross part of the handle. This figure is on a scale of two lines to an inch, English measure.

In fig. 2 the essential parts of the instrument are shown on a scale of four lines to an inch.

A is a glass bottle, holding two or three centilitres, the mouth of which is ground perfectly even; and confined by the screw d in the collar B, which opens with a joint at C,

Fig.

so that the vessel may easily be taken out, to be cleaned or weighed.

E the cover, to which the glass stopple is cemented. It is here seen closed by the position given to the counterpoise F, which is secured by the pressure of the catch g, in the movable part of the counterpoise H, on the elbow in the handle.

VII.

The Croonian Lecture. By WILLIAM HYDE WOLLASTON,

I AM

M. D. Sec. R. S.*

AM aware that the remarks, which I have to offer on the Croonian present occasion, may be thought to bear too little direct lecture. relation to each other for insertion in the same lecture; yet any observation respecting the mode of action of voluntary muscles, and every inquiry into the causes which derange, and into the means of assisting the action of the heart and blood vessels, must be allowed to promote the design of Dr. Croone, who instituted these annual disquisitions.

And it has always appeared to be one great advantage at One of the adtending the labours of this society, that it favours the pro- vantages of the society. duction of any original knowledge, however small, in a detached form; and enables a writer to say all that he knows upon a particular subject, without inducing him to aim at the importance of a long dissertation.

I shall therefore make no apology for dividing the follow- Subjects of ing lecture into three distinct parts. this paper.

In the first of which I shall treat of the duration of volun

tary action.

In the second, I shall attempt to investigate the origin of seasickness, as arising from a simple mechanical cause deranging the circulation of the blood.

In the third, I shall endeavour to explain the advantage. derived from riding and other modes of gestation, in assist

* Philos. Trans. for 1810, p 1.

VOL. XXVII.-DEC. 1810.

U

ing