On comparing this with Mr Anderson's results, Edinburgh Encyclopædia, art. Hygrometry, a remarkable disagreement will be perceived, both as to the quantity of the depression, and the rate at which it is influenced by pressure. The following are the results which Mr Anderson obtained by placing Leslie's hygrometer under a receiver along with sulphuric acid; the temperature of the air being 48°.5 Fahrenheit. The first column is the pressure; the second the depression in degrees of Leslie's hygrometer, which, for the sake of comparison, I have reduced to degrees of Fahrenheit in the third. The temperature and pressure in Mr Anderson's first case, are nearly the same as mine, but our depressions are very different; his being only 4°.86, whilst mine is 11°.6, which is more than twice as great. This discordance led me, at first, to suspect, that as, in Mr Anderson's experiments, the wet ball of the hygrometer would, from its construction, be six or eight inches above the surface of the acid, whilst in mine it did not exceed one inch, this might be the reason why his depressions were so small. But on trying this, the result was 49°—39°-10°, still double of Mr Anderson's numbers; even though the surface of sulphuric acid did not exceed half of that in the former experiments, so that this does not appear to have been the reason why Mr A.'s numbers are so small. Indeed I have repeatedly obtained greater depressions than 4°.86, by merely suspending the instrument in a room where no means were used to dry the air, or raise its temperature *; such as 46°.5 — 40°.5—6° ; 45°8.-40.°5-5°.3; 47°.5-41°-6°.5. The barometer was rather higher than Mr Anderson's; and had the temperature been raised to 48°.5 Fahrenheit, the depression would have been a little increased. The difference surely could not proceed from any defect in For, as is well known, very cold air, by being heated, without additional moisture, becomes comparatively dry. Mr Anderson's air-pump, as I understand he has an excellent one, and knows as well how to use it. But it is curious that he seems scarcely to have reached the freezing point, even under greater exhaustions than I have yet employed. My experiments were made by a very powerful double barrelled air-pump, made by Mr Dunn, optician in Edinburgh, a very ingenious artist, who, to great practical skill in the workmanship, joins a corresponding acquaintance with the scientific principles of his profession. The barrels of his pumps are considerably larger than those commonly made in London; so that a few turns of the handle can freeze the wet thermometer under a receiver perfectly white. Most air-pumps are very defective in not having the plate ground truly flat. This, indeed, is reckoned so easily done, that it is too often neglected, to the great detriment of the instrument. The attention of Mr Dunn to this most important part of an air-pump, forms no small recommendation to his instruments; though, I believe, he is equally careful in the execution of all his work. Since the foregoing account was written, I have made another set of experiments on the effects of pressure at rather higher temperatures. The following are the results: Here, as before, the first column is the pressure; the second the temperature of the dry thermometer; the third that of the moist, and the fourth the depression. The greatest exhaustion is here the same as Mr Anderson's, but the temperature of the moist ball is somewhat lower, even though the dry one be 10° higher than his. The depression, in the fourth column, follows a law very different from the reciprocal of the pressure. The conclusion drawn by Mr Anderson from his experiments is, that, in air of the same dryness and temperature, the depression is inversely as the barometric pressure. Mr Ivory again, from his investigation, Phil. Mag. lx. 85, has brought out a very different result, that when the temperature, not of the air, but of the moist bulb, is the same, the depression is inversely as the pressure. This, no doubt, comes much nearer to my results than to those of Mr Anderson, though it is not very easy to make the comparison, on account of the different temperatures of the moist ball, at the various pressures in the foregoing tables. I must not omit to mention, that, in these tables, the temperatures themselves still require a small correction; because the thermometers were, as is usual, sealed or close at top, and would, therefore, stand a little too low when under the reduced pres sure. For, on placing them in a dry state, under a receiver, and exhausting to the utmost, both stood 1°.5 Fahr. lower when the former temperature was restored. Hence, as the entire barometric pressure is to the reduction of pressure, so is 1°.5 to the correction sought. Other thermometers put in with them did not all undergo the same change. For this, there are no doubt various reasons. It is easily shewn, that, within a moderate range, the error will, cæteris paribus, be nearly as the change of pressure, multiplied by the diameter of the bulb, divided by the thickness of the glass. But the sinking of the dry thermometer a little, in these experiments, was partly the influence of the cold wet ball on the still confined air. It has been long known, that thermometers were affected by pressure; and to avoid this, a very effectual method, when applicable, was adopted by Professor Leslie, who employed thermometers open at top, when he had occasion to use them under a variable pressure. Some, however, give themselves no concern about the matter. In experiments on the force of steam, the ball of the thermometer is often included in the boiler with the stem projecting outward. The pressure on the ball may then vary from a small fraction of an inch of mercury to many atmospheres; and, in such cases, the temperatures must be erroneous enough Mr Crichton has already pointed out some serious oversights of a different kind in the Memoir of MM. Dulong and Petit on Large flasks and receivers, if thin, must have their capacities somewhat altered, by varying the pressure. This alteration in similar shaped vessels, will be, cæteris paribus, nearly as the fourth power of the diameter divided by the thickness. Expansions*; and I have some suspicion, that, in their very elaborate experiments on the cooling of large thermometers, they have overlooked the influence of change of pressure; the effects of which were the more to be feared, on account of the gigantic size of the bulbs, and the great range through which they ope rated. The glass of large thermometers is usually thinner, especially in proportion to their diameters, than of small ones; and if it was so in their case, the errors would be so much the greater; but these learned authors have given us no data from which the amount of such an effect could be estimated. This, however, they might still do, if the instruments be preserved. 57 Farther experiments are perhaps wanted, regarding the depression of wet thermometers; but at present, I may mention that the two sets which I have given above, especially the first, make the depression, through a range which will seldom be exceeded, nearly proportional to where B is the height of the barometer in inches; and probably a still more exact number might be found, by which the observed depression being divided, will be reduced to what it would have been under the standard presAs a temperature of 60° rarely occurs at great elevations, the last table is not suited to their case; and, therefore, its deviating a little from this formula, when the pressure is small, becomes a matter of no moment. From these experiments, it appears, that the variations of pressure have much less influence on evaporation than is commonly supposed; and that, on the same spot, variations of atmospheric pressure may, without much danger of error, be neglected. sure. According to Professor Leslie and Mr Ivory, the depression of the moistened thermometer, under the same pressure, is proportional to the drying quality of the air after its temperature is so reduced. Or, a given volume at that reduced temperature, can still retain c d more grains of moisture than is already contained in the like bulk of surrounding air; c being a constant coefficient to be determined by experiment. Hence, if = actual weight of moisture in the given volume, at the existing tem * Annales de Chim. et de Phys. tome vii.; Annals Phil. xiii.; and for Mr Crichton's remarks, see Annals Phil. xxiii. perature of the air t, and u the maximum at the temperature m of the moist bulb; also t―m being=d, we have w=u cd. But if the temperature 7, at which w grains would saturate the original volume, be wanted, it may be found from the thermometers only, without the aid of any tables, by the following approximate formula, which, however, comes very close to the foregoing, between the temperatures of 25° and 90° Fahrenheit. Put k for the temperature at which the variation in the weight of moisture in the given volume for a change of 1° is c grain, then the temperature sought will be If the volume be a cubic foot, and if, as appears from a mean of various experiments, c=.15, then k= 53° Fahr., and If the centigrade thermometer be used, c.27, and both k and m must be increased about 18°. Hence The maximum forces of vapour for different temperatures follow a law very similar, and nearly related, to the law of the density. So that the actual force of vapour in the air may be represented by ƒ=F-gd; where F = maximum force at the temperature m, and g a constant, which will .0125 or 1 when c=.15. Hence the temperature at which aqueous vapour having the force f, would be in a state of saturation, and which temperature is usually called the dewing point, will be 8 The number substituted for k in this case being 49°.5 Fahr. the temperature at which the variation of force for 1° is .0125. By means of this formula, the point of deposition, or dewing point, may be readily obtained without the aid of tables. With the centigrade thermometer, These formulæ are adapted to the ordinary pressure, and are by much the simplest I have ever seen for the purpose. The dewing point, or point of deposition, is the temperature of saturation under the original pressure. The temperature is |