28 Mr Henry Meikle''s Remarks and Experiments 



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 corres- 

 ponding acquaintance with the scientific principles of his profes- 

 sion. 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 perfect- 

 ly 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 in- 

 struments ; though, I believe, he is equally careful in the exe- 

 cution of all his work. 



Since the foregoing account was written, I have made another 

 ^et of experiments on the effects of pressure at rather liigher 

 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, foUows 

 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 depres- 

 sion is inversely as the barometric pressure. Mr Ivory again, 

 from his investigation, Phil. Mag. Ix. 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 



