notation are those employed by Encke in his Memoir on the Method of Least Squares.

The measure of precision (h), as was indeed to have been expected, decreases with the temperature. This fact is not, however, of so much importance as might at first appear; for the dew point is given by the equation

T=t-f(t-t');

where T is the temperature of the dew point, t that of the air, (t-t') the difference between the dry and wet bulb thermometers, and f the factor, whose value is given in the table.

Now taking the temperatures 42° and 22°, it appears from the table that the probable error of f, from a single observation, is at the latter temperature three times as great as at the former, but (t-t') is, on an average, about three times as great at 42° as at 22°. Hence the probable error of the dew point at both temperatures is very nearly the same.

purpose of comparison

We have extended our table to 51° for the with the "Greenwich factors." I must however remark that it is probable that the factors above 40° are rather greater than they would have been had the observations discussed extended through a longer space of time, the majority, at these temperatures, having been taken last spring, when the air was very remarkably dry; and experience shews thatwhen (t-t') is unusually great the deduced factor, instead of being more accurate, is generally much too large.

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As an instance I may cite an observation taken on the 21st April, when the temperature of the air was 43.6°, that of evaporation was 31.69, and that of the dew point was 3.2°. The fraction of saturation on this occasion was only 10%, and the factor derived from this observation was 3.4, being much the largest deviation from the adopted mean, 2.53. The cause of this discrepancy is doubtless owing to the heat that the wet bulb thermometer derives from the radiation of surrounding objects, and, were observations sufficiently numerous, it might conduce to accuracy were the factors calculated for every degree of difference in the value of (t-t').

We purpose instituting a comparison between two wet bulb thermometers placed in similar boxes, the one coated with lampblack and the other with polished daguerreotype plates.

Below 32° our results do not appear to coincide with the factors deduced from the Greenwich observations, and the cause of the discrepancies must be left to time. As, however, we have had considerable experience at these temperatures, I may perhaps be doing service to observers in bringing before their notice two causes of error, to which we have found ourselves particularly liable when the thermometer is

near 320-1st. If the air is a little above, and has been below 32°, there will frequently be a small button of ice at the foot of the wet bulb thermometer which is not easily perceived, and which will keep it at 32° when the temperature of evaporation is really above that point. 2nd. It is well known that under certain circumstances water may be cooled below 32° without freezing, and an example will perhaps best shew the error which this may occasion. Let us suppose that the temperature of the air is 27°, and that when the thermometer is wetted it sinks to 26° and then rises. Should it rise very slowly the probability is that 26° is the true temperature of evaporation, but if rapidly, the rise may be due to the conversion of the water into ice, and it will be prudent to observe whether or not the thermometer again commences to sink. We have frequently observed this phenomenon, and I am quite at a loss to what to ascribe its uncertainty. It has occurred both in a high wind and a calm, (the thermometers are protected from the full force of the wind,) and it also appeared to be quite uncertain at what temperature the water would freeze.

I am obliged to admit that the limits of certainty of the factors below zero are not so close as could be desired. This is partly attributable to our having to reject many observations made with a thermometer which was broken before its index errors were fully ascertained. Mr. Campbell and I must claim the indulgence of those who know the difficulty of taking observations requiring so much time and accuracy, at such temperatures, and frequently at six o'clock in the morning.

NOTES ON SOME POINTS IN THE ANATOMY OF THE LEECH.

BY JAMES BOVELL, M. D.

PROFESSOR OF THE INSTITUTES OF MEDICINE, TRINITY COLLEGE, TORONTO.

Read before the Canadian Institute, December 15th, 1855. Dugès, Home, Jones, and other distinguished anatomists, in their descriptions of the structure of the Leech have assigned to certain highly developed parts in this Annelid, functions which it was by no means clear to many more recent observers, could legitimately be performed by them. It was reserved, however, for Dr. Williams, of Swansea, a highly distinguished comparative anatomist, to unravel the mystery, and to furnish proof of the errors into which his predecessors had fallen.

The existence of an elaborate circulating system seemed to necessitate an ærating one equally developed in character; but spiral vessels, on the type of insecta, no where being seen, the vascularwalled pouches, occupying the lateral regions of the body, seemed to be the organs of respiration, supplied freely with blood by vascular hearts. While many doubted the existence of so special an organization for respiration in this creature as was described, no one before Dr. Williams had assigned them to the generative apparatus, and as I believe that the observations which have been repeated here confirm the results arrived at by the Naturalist of Swansea, I thought it of sufficient interest to bring the subject before the Institute. I cannot, however, agree with Dr. Williams that the generative organs are rightly described, even by himself. In order to understand the subject, as now unfolded to us, it may be more advantageous to state the opinion of one of the highest authorities.

Mr. Jones, in his "Animal Kingdom," observes: "Two lateral vessels are appropriated to the supply of the respiratory system, and in them the blood moves in a circle quite independent of that formed by the dorsal artery and ventral vein, although they all communicate freely by means of cross branches, those passing from the lateral vessels to the dorsal being called by Dugès dorso-lateral, while those which join the lateral trunks to the ventral canal are the lateroabdominal branches. The movement of the blood in the lateral or respiratory system of vessels is quite distinct from that which is accomplished in the dorso-ventral system or systemic.

On examining one of the respiratory pouches, its membranous walls are seen to be covered with very fine vascular ramifications, derived from two sources; the latero-abdominal vessel gives off