and mixed with the secretions poured out by the glandular surfaces surrounding it, in this respect corresponding to the first and second stomach of ruminating animals, in which the grass is prepared for mastication.

These facts, as they account for the goose and swan living upon a species of food not fitted for birds in general, proves that in this class of animals, a peculiar conformation of the digestive organs is required, as well as in quadrupeds, when they are intended by nature to live intirely upon grass.

This fact being ascertained, is not without its importance in elucidating this branch of physiological inquiry.

EXPLANATION OF THE PLATE.

Fig. 1. The gizzard of a turkey laid open on its anterior part, to shew the form of the internal cavity, which is oval, and the grinding surfaces uniformly concave.

Fig. 2. The gizzard of a swan exposed in the same way; the grinding surfaces have an oval form, but in an opposite direction to those of the turkey, and each of them is made up of a ridge and a hollow in the direction of the oval, which are adapted to those of the opposite side, the ridge of the one fitting the hollow of the other.

X. Observations on Atmospherical Refraction as it affects astronomical Observations; in a Letter from S. Groombridge, Esq. to the Rev. Nevil Maskelyne, D. D. F. R. S. Astronomer Royal. Communicated by the Astronomer Royal.

DEAR SIR,

Read March 28th, 1810.

Blackheath, 22nd January, 1810.

THE great advantages we derive from the labours and writings of those learned men, who have preceded us; the profound researches in physical astronomy by the mathematicians of the last century; and the improvements in the construction of instruments, effected by the ingenuity of our modern artists: these circumstances should stimulate all those who have inclination and opportunity, to the endeavour of contributing to the common stock of knowledge. I beg leave to add my tribute of admiration to the general voice, for the large share you, Sir, have taken in the promotion of science, not only by your learned publications, but also by the liberal encouragement and assistance you have given to those who have been honoured with your acquaintance. I am, therefore, induced to commit to your protection the inclosed paper, acknowledging myself indebted to your friendly communications for much useful information. I could have wished the task had been performed at an earlier period; but as you are well acquainted with the labour required in the reduction of

so many observations by one person, it will appear to have

been impracticable.

I remain,

Dear Sir,

Your obliged and very obedient servant,
STEPHEN GROOMBRIDGE.

WHEN I had fixed my four feet transit circle, made by TROUGHTON, it was my intention to make observations of a sufficient number of circumpolar stars, at different altitudes, to ascertain the latitude of my observatory with the greater precision; and also to endeavour to settle the true quantity of refraction: especially as my instrument was better adapted -for that purpose, both from the construction and convenient size thereof, than those which had been heretofore made. Being fixed on stone piers, it is not so liable to partial expansion, as those instruments which are supported on brass frames: and having both sides divided, with two microscopes to each face; the same observation has the advantage of four made with a quadrant. I have also contrived, by means of sliding shutters in the roof, to prevent the rays of the sun from falling on any part, excepting on the object glass. By these means I flatter myself that, the observations having been made with great care and attention, the result will prove both satisfactory and useful.

Having proposed to myself the above course of observations, I selected the fifty stars contained in the first table for that purpose, the observations of which, exceed in number