Page:Proceedings of the Royal Society of London Vol 1.djvu/76

Rh chiefly of a-continuation of his observations on the appearances of that body from the 2nd of March to the 3rd of May last.

Conceiving that there might be some advantage in getting rid of the darkening glasses in viewing the sun, he was led to substitute for them various liquors, such as spirits of wine, port wine, ink diluted with water, a solution of green vitriol with a small proportion of tincture of galls, and even plain water; which latter he found keeps off the heat so effectually, that the brightest sun may be viewed some time through it without any inconvenience.

Through diluted ink, the image of the sun appeared as white as snow; and when the liquor was still more diluted, the sun was of a purple hue, while the objects on its surface continued as distinct as when seen through any other medium. From these observations the author infers that the continuance of the symptoms which in his former pаper he considered as favourable to the copious emission of light and heat from the sun, are sufficiently verified, and that by comparing these phænomena with the corresponding mildness of the season, his arguments respecting the connexion between them and the temperature of our atmosphere acquire no small degree of pro- bability.

Being well aware that the price of wheat which he adopted in his former paper as a criterion of the seasons is liable to some objections, the author desires here to be understood, that his intention was merely to compare the astronomical fact of the variable emission of the sun's rays with the obvious symptoms corresponding with that circumstance; leaving it to others to apply the subject to such use- ful oeconomical purposes as may be found to have any relation to them: at any rate, he cannot relinquish the hope that astronomy will ultimately supply us with the means of deriving certain prognostics of the temperature of the seasons from accurate observations on the quantity of the light we receive from the sun.

The great utility of Hadley's quadrant in practical astronomy, and particularly in navigation, has given rise to several improvements of that valuable instrument, of which some account is premised in the present paper. The first of these is due to the celebrated Tobias Meyer, who, by completing the limb of the sextanr into a whole circle, and adding an horizon index, enabled us to repeat the obser- vations, so as to ascertain the double, triple, and even a greater multiple of the angles; by which means the errors of division or eccentricity in the instrument can be reduced in the inverse ratio of the repetition of the observations, so as to arrive at any degree of approximation that may be required.

Some imperfection still remaining as to the manner of renderimg the glasses parallel, so as to produce the exact coincidence of the images, the Chevalier de Borda contrived a method of rendering this exact parallelism of less consequence, by substituting the immediate of the sextant into a whole