NATURAL PHILOSOPHY. 117 



"The phantom-images of the retina, as I have remarked, are not 

 perceptible in the light of day. Those that exist in the sensorium, 

 in like manner, do not attract our attention so long as the sensory or- 

 gans are in vigorous operation, and occupied with bringing new im- 

 pressions in. But when these organs become weary and dull, or when 

 we experience hours of great anxiety, or are in twilight reveries, or 

 asleep, the latent apparitions have their vividness increased by the con- 

 trast, and obtrude themselves on the mind. For the same reason they 

 occupy us in the delirium of fevers, and doubtless also in the solemn 

 moments of death. During a third part of our lives we are withdrawn 

 from external influences, hearing, and sight, and the other senses 

 are inactive ; but the never-sleeping mind, that pensive, that veiled 

 enchantress, in her mysterious retirement, looks over the ambrotypes 

 she has collected, ambrotypes, for they are unfading impressions, 

 and combining them together as they chance to occur, weaves from 

 them a web of dreams. Nature has thus introduced into our very 

 organization a means of imparting to us suggestions on some of the 

 most profound Copies with which we can be concerned. It operates 

 equally on the savage and on the civilized man, furnishing to both 

 conceptions of a world in which all is unsubstantial. It marvelously 

 extracts from the vestiges of the impressions of the past overwhelm- 

 ing proofs of the reality of the future, and gathering its power from 

 what might seem a most unlikely source, it insensibly leads us no 

 matter who or where we may be to a profound belief in the immor- 

 tal and imperishable, from phantoms that have scarcely made their 

 appearance before they are ready to vanish away ! " 



APPARATUS FOR MEASURING THE VELOCITY OF PROJECTILES. 



The following is a description of an electro-ballistic apparatus, re- 

 cently invented by Major Navcz of the Belgium army for measuring a 

 very small space of time, such, for instance, as a cannon-ball would 

 take in passing over a few yards. Before we proceed to explain this 

 interesting machine, it will be necessary to remind our readers that an 

 electric current has the property of magnetizing soft iron, and also, to 

 mention the peculiar principle of the pendulum. This is, that the 

 same pendulum will always, within certain limits, perform unequal 

 vibrations in equal times, that is to say, that a "seconds' 1 pendu- 

 lum will always take a second to make one oscillation, whether it be 

 raised from the perpendicular 20 or 5. Consequently, supposing 

 a seconds pendulum to be selected we can take the arc it describes in 

 one vibration, and, by dividing this arc into a scale of parts, we can 

 arrest the pendulum as it falls ; and the distance it has fallen, measured 

 into the whole length of the arc, will give the fraction of a second in 

 which the fall took place. We see then, with what extreme minuteness 

 we can measure time by stopping the seconds pendulum before it has 

 fallen the y~oYo part of an arc in which it would vibrate. The electro- 

 ballistic apparatus is used for determining the velocity of a projectile, 

 or the rate at which a shot proceeds after it leaves the muzzle of a 

 gun. The instrument consists of three separate parts, one of which 

 is a principal and the others accessories. The chief part consists of 

 a graduated arc, on which a pendulum is so adjusted that it can be ar- 

 rested at any period of its oscillation, and thus denote the time it has 



