Rain, by the pluviameter, between noon the 1st of Feb. and noon the 1st of March, 2-828 inches. Evaporation, during the same period, 1.430 inch. **The Editor has been requested by Mr. Adams to insert the following note: Stonehouse, March 13, 1819. I have this day been informed by a friend, that my method "for clearing the Lunar Distance," published in your journal for this month, is essentially the same as one given by Captain Robert Heath in the "Supplement to the Royal Astronomer," published in 1768; and I, therefore, hasten to acquaint you of the circumstance, and to beg the favour of your inserting this in Annals for next April. I have never seen the work your entitled the "Royal Astronomer," nor heard, either directly or indirectly, of Capt. Heath's method. The publication of this note will, I hope, remove any idea of my having borrowed the principle. ERRATUM in the same Paper, In No. LXXV, p. 191, line 23, for 54° 43' 20' read 58° 43' 20". ANNALS OF PHILOSOPHY. MAY, 1819. ARTICLE I. Researches on the Measure of Temperatures, and on the Laws of the Communication of Heat. By MM. Dulong and Petit. (Concluded from p. 251.) Of Cooling in the Air and in Gases. THE laws of cooling in vacuo being known, nothing is more simple than to separate from the total cooling of a body surrounded with air, or with any other gas, the portion of the effect due to the contact of this fluid. For this, it is obviously sufficient to subtract from the real velocities of cooling those velocities which would take place, if the body, cæteris paribus, were placed in vacuo. This subtraction may be easily accomplished now that we have a formula which represents this velocity with great precision, and for all possible cases. We can then determine the energy of cooling due to the sole contact of fluids, and such as it would be observed directly if the body could be deprived of the faculty of radiating. This part of our labour required a very considerable number of experiments, because the laws which we wished to discover were to be studied with respect to the different gases, and for each of them at different temperatures, and under different pressures. Each experiment was made and calculated as we have explained above. We shall, therefore, satisfy ourselves with stating the mean results of these different observations. The first question with which we behoved to occupy ourselves, was to ascertain whether the modifications of the surface of VOL. XIII. N° V. bodies, which produce so powerful an influence on the radiation, occasioned any change in the losses of heat occasioned by the contact of fluids. For this it was sufficient to observe the cooling of our thermometer in a gas of a determinate elasticity and temperature, first with its bulb in the natural state, and then covered with a leaf of silver. Of all the experiments which had this comparison for its object, we shall only give the two following. In the first, we observed the cooling of the largest of our thermometers in the balloon containing air under a pressure of 0.72 metre, and at the temperature of 20°. I་ ་་ First Case.-The thermometer being in its natural state. Excess of temper-Total velocities of ature of the ther mometer. Velocities of cool the cooling of the ing which would Velocity of cooling due to the air 7 1 thermometer. have taken place alone. in vacuo. Excess of temper-Total velocities of the Velocity of cooling Velocity of cooling ature of the ther cooling of the therm. in vacuo. due to the air. mometer. 200° We see, by comparing the last columns of the two preceding tables, that the corresponding numbers deviate so little, that ther deviation may with propriety be ascribed to errors in the expe-i riments. Air then, other things being the same, takes away the same quantity of heat from vitreous and metallic surfaces. The two following tables contain all the elements of a similar comparison made with hydrogen gas. The small thermometer in these experiments was substituted for the large one. The experiments were made at the temperature of 20°, the gas being subjected to a pressure of 0-74 metre. This comparison gives for hydrogen a result similar to that for air. The equality being thus verified for surfaces differing so much from each other as glass and silver, and for gases of such different qualities as air and hydrogen, it is natural to generalize the result, and to deduce from it the following law. The loss of heat owing to the contact of a gas, other things being equal, is independent of the state of the surface of the body which cools. This remarkable law of the communication of heat has been already admitted by Mr. Leslie. But this skilful philosopher has only given it as a probable consequence of two indirect experiments, which consist in proving that the state of the surface has only a very feeble effect on the time of cooling in those circumstances in which radiation can contribute but a very small portion of the loss of heat. This is the case, for example, when a hot body is exposed to a violent wind, or when it is plunged into a liquid. But these experiments, however ingenious, can never completely supply the place of direct observations. And in the present case would it not have been possible, for example, to suppose that a property observed in air while violently in motion, could only be applied in a limited sense to air in a state of rest? This doubt would appear still better founded, or would be changed into certainty, if we admitted with Mr. Leslie that air in a state of rest deprives bodies of heat by two different ways; namely, by a conducting property such as exists in solids, and by the renewal of the fluid from ascending currents. Our process, by enabling us in the first place to show the existence of the same law in different gases, dissipates all the doubts. which the experiments of Mr. Leslie still allowed to remain. This is one of the cases in which the advantages of the uniform method which we have adopted can be best seen. |