Scient'ijic Intelligence. — Natural Philosophy. 179 



the mixed salts, we are ready for finding the dew-point in all si- 

 tuations, and a very <ew minutes will suffice for the purpose. 

 Within-doors, we have only to call for a jug of water, and a 

 small cup to receive the liquid used in rincing out the brass 

 ball once or twice after the operation. In the open air, a spring 

 or brook will supply all our wants. In .situations where water 

 may not be found, as on the summit of mountains, a phial of 

 the liquid carried with us will remedy the deficiency ; and if its 

 temperature has risen a little too high, we have only to employ 

 1^ or 2 measures of the salts, as may be necessary. The writer 

 of this note has, for a year or two past, used this little apparatus 

 in a great variety of situations, and, amongst others, during the 

 barometric measurement of heights ; and having thus had suffi:- 

 cient experience of its usefulness, he has thought that it might 

 be worth while to give this short notice respecting it. 



During frost, water at S'6\ and 1, 1^, or 2 measures of salts, 

 as circumstances may point out, will give the requisite cold. 

 For more severe climates, the vessel might be of polished plati- 

 num, and the materials an acid and ice, or the proper salts. 



A"^. CoNNELL. 



SCIENTIFIC INTELLIGENCE. 



NATCKAL PHILOSOPHY. 



1. Velocity of Electricity.— The following notice of Profes- 

 sor Wheatstone's very interesting experiments on the velocity of 

 Electricity, will enable the reader to form a general conception 

 of them. Two very great difficulties present themselves in this 

 inquiry, the one arising from the circumstance that the sensa- 

 tion of light on the retina continues longer than the impression 

 which produces it, as is illustrated by the well known experi- 

 ment of a luminous circle being produced by a point of light in 

 rapid rotation; this prolongation, very manifestly, is a great 

 obstacle to the exact measurement of the duration of the im- 

 pression : the second difficulty is the prodigious extent of any 

 conductor that could be used for the purpose of exhibiting any 

 appreciable difference in the transit of a spark from one point to 

 another. The immense velocity of electricity makes it impossi- 

 ble to calculate it by direct observation ; it would require to be 



m2 



