Mr. Tovey's Reply to Mr. Potter. 431 



depend on the angle which the direction of the disturbing force 

 makes with that of the regular force. By the hourly observa- 

 tions, the 23rd of March appeared to be a day of greater 

 disturbance than any other in the month in respect to the hori- 

 zontal intensity, although not a single shock was shown by the 

 declination magnetometer. Had, however, the observations 

 been continuous instead of hourly, the declination might also 

 have been seen to have been disturbed. 



VII. The more these phenomena are studied the more 

 strong becomes the impression of the importance of observing 

 them at short intervals. Those of 5 minutes are too long, for 

 the bar may alter its position many hundred scale-divisions 

 in that interval, as we found by the Prague bifilar on the 

 22nd of October. On days of great disturbances, or when 

 the aurora borealis is seen, we observe the two horizontal 

 elements, here and at Gbttingen, uninterruptedly for several 

 hours at intervals of 15 or 20 seconds: it is much to be de- 

 sired that this practice should become more general. 



In my letter to M. Kupffer, I noticed certain vertical vi- 

 brations supposed to be produced by earthquakes. By a 

 mistake of the pen these were said to have taken place in the 

 dipping-needle, whereas it was in the declination magnetome- 

 ter that they occurred. In Prague this instrument is sus- 

 pended to a beam, which is supported by the principal walls 

 of the house, and such vibrations only take place during 

 very violent winds. In such cases there can be no doubt 

 that they are due to this mechanical cause. 



In addition to the magnetic observations, we now note the 

 temperature of the earth from one to five feet deep, and the 

 height and temperature of the Moldau. I wish to add ob- 

 servations on the temperature of the dew-point and the in- 

 tensity of the sun's rays, and shall be much obliged to you 

 to inform me the price of the actinometer and the dew-point 

 hygrometer, that I may request our government to purchase 

 these instruments. 



I remain, with the highest esteem, &c. &c. 

 Major Sabine, R.A. * Carl Kreil. 



LXII. On Mr. Potter's Application o/'Huyghens's Principle 

 in Physical Optics. By John Tovey, Esq. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN your October Number, p. 243, Mr. Potter gives some 

 examples of the application of the principle above-named; 

 from which he contends that the result of this principle is 



