would prevent their either being carried away bodily, or the index from being disturbed by unavoidable wind or still more provoking curiosity. As to the first point, I have gratefully to acknowledge the assistance of a large number of our best mountaineers, who have either deposited instruments themselves, or sent reports of the readings of those already placed. Thanks to their united efforts, about thirty minimum thermometers have been exposed at altitudes of from 7150 to 15,784 feet over a wide tract of country, extending from the summits of the Viso, Grand Pelvoux, to the Marmolata in the Southern Tyrol.

"(2) The correctness of the instruments was, as far as possible, secured by entrusting their construction to Mr. L. P. Casella, one of our best makers, and their uniformity by the adoption of a definite pattern. At first, in the absence of a good mercurial minimum capable of acting in a horizontal position, and in the uncertainty as to the sufficiency of the range of mercury, the ordinary spirit- or Rutherford's thermometer was adopted; but experience has in the great majority of cases demonstrated its inefficiency, and in consequence all the instruments deposited during the past summer and autumn, four in number, have been mercurial, of Casella's last patent construction.

(3) The question of exposure has not been solved as satisfactorily as could be desired, and this failure has I fear destroyed much of the value of the results obtained. In the first place, the process of attaching a thermometer to a bare rock at great elevations, often in a keen frost and chilling wind, is by no means so easy as the enthusiastic meteorologist may suppose; and without discussing here the various precautions which ought to be, and perhaps might be, adopted in some exceptional cases, I would venture to express an opinion that a well-constructed cairn of sufficient elevation, so placed as to prevent its being buried by winter snows, is the simplest and most efficient means of protecting the thermometer from the most serious causes of disturbance. When, at least, this plan has been adopted, the readings of the instruments have appeared trustworthy, and in almost all other cases sadly the reverse. By this means also they are more screened from inquisitive observation, and may better escape the pilfering propensities of an inferior order of guides, whom we probably have to thank for the disappearance of one at least fixed very securely by the writer on the Aiguille du Goûté.

"A large proportion of the Rutherford minimums have become perfectly useless from the division of the column, and it is this fact, coupled with a belief that the lowest temperature of winter on the loftiest summits rarely exceeds -40° Cent. (the freezing-point of mercury), which has led to their abandonment and the substitution of the mercurial construction. From some recent experiments, consisting in the alternate exposure of spirit-minimums to varying temperatures, I am disposed to attribute the separation of the column to this cause, which, if due precautions are not observed in placing the instrument, must be especially energetic at great altitudes.

"Unless the thermometer can be protected from the influence of radiation at night, or the respectively cooling and warming effects of a thin or thick layer of snow, variations from the true temperature of the air, amounting (as shown by M. Martins) to 10° or 12° Cent. (18° to 22° Fahr.), may be produced, and the reading utterly vitiated for purposes of comparison. Besides, if imperfectly shielded from radiation, it will probably be more or less subjected to the direct action of the solar rays, and thus be exposed to temperatures varying within twenty-four hours by as much as 55° C. (100° Fahr.). My experiments show that a much more limited range than this suffices to

produce a solution of continuity' in the column of spirit, which has acquired amongst our mountaineers the expressive name of the bubble complaint.'

"In one instance an observer, whose accuracy I have no reason to doubt, informs me that he could detect no trace whatever of spirit, nor any indication of fracture in the glass by which it could have escaped. The index lay 'high and dry' at the bottom of the bulb. This extraordinary result he attributes to 6 a sort of volatilization of the contained spirit;' and though it seems difficult to understand how it could have taken place to the extent mentioned, there is little doubt that vaporization of the contained spirit to an extraordinary extent will occur, as pointed out by Dr. Hooker some years ago in the Appendix to his Himalayan Journal.' If my informant's statement appear exaggerated, I hope the probable truth which underlies it may draw attention to the question.

"The causes just alluded to, and the comparatively short time which has elapsed since these observations were commenced, must be accepted as some justification of the meagreness of the results.

"The readings of the minimum temperature of the autumn and summer months at elevations of 9000 to 15,000 feet (Table III.) appear rarely to fall below -10° Cent., or if they do, the condition of the thermometer is generally stated by observers to be suspicious. The lowest winter reading registered is -41° C., in the case of a thermometer placed on the Col d'Argentière at a height of upwards of 12,000 feet; but as when observed the spirit had separated, we have no right to assume that it had not done so before the index attained its actual position. We have, however, four observations which seem entitled to entire confidence as far as the instrument is concerned, though one at least certainly does not represent the lowest temperature of the air. The minimum on the Becca di Nona, near Aosta, carefully deposited in a cairn at a height of 10,382 feet, has been found in perfect working order after the lapse of two years. My excellent friend M. Carrel informs me that the minimum temperature of the winter of 1860-61 and 1861-62 was respectively - 27° and 23° C. (— 17° and 10° Fahr.). Again, a similar instrument on the Col d'Erin, at a height of 11,408 feet, was found in perfect preservation by Mr. Whately last autumn after exposure during one winter, that of 1860-61. Its minimum reading was 21° C. (— 6° Fahr.); but as earlier in the season I was unable to find it, though it had been deposited by myself in 1860, there is no doubt that it must have been buried in the snow during either the spring or winter, and thus its indications are probably considerably too low, since for the same period the temperature on the Becca di Nona (1000 feet lower) fell to 27°. Lastly, a thermometer placed last year in a cairn on Scaw-Fell Pike appeared to be in good order this spring, and registered — 10° C. (+ 14° Fahr.) as the greatest winter cold.

"To the above observations it may not be amiss to add one by M. Lizat on the Pic de Nethou, the highest point of the Pyrenees (11,168 English feet). This instrument, placed at the summit, registered 24°.2 C. in the winter of 1857. If we compare the preceding observations with the registers kept at Geneva and the Great St. Bernard, we have during the winter 1859-60 at Geneva the minimum readings of 23° on 21st Dec. 1859, and - 11°1 on 16th February 1860. Corresponding to these, the lowest temperatures recorded at the Great St. Bernard were 27°.2 on 16th December 1859, and 25°.3 on 10th March 1860. Even allowing that we are not certain that the instruments at levels higher than the Great St. Bernard were clear

Observer's Name.

TABLE II.-Observations of the Temperature of the Soil at and near the surface.

of winter snow, there is at least reason to suspect that the proportionate fall of the thermometer with increase of height is much less considerable in winter than at other seasons."-F. F. TUCKETT.

2. The objections to attempting a measure of the radiant heat of the sun by exposing black-bulb thermometers, are obvious and well known; nevertheless it was thought that by using instruments as nearly as possible identical in construction, exposed in the same manner, and rejecting all observations in which the result could be affected by wind, results comparable inter se might be obtained. It is believed that if the first condition could be secured this inference would be found correct, but in point of fact it is a matter of extreme difficulty to obtain the requisite identity of construc