Polytechnic Association. 581 



made completely impervious to rain, and, moreover, I have learned 

 how to make it so ; and for the benefit of my readers, I will here 

 give the recipe : In a bucket of soft water put half a pound of sugar of 

 lead and half a pound of powdered alum; stir this at intervals 

 until it becomes clear ; then pour it off into another bucket, and put 

 the garment therein, and let it be in for twenty-four hours, and then 

 hang it up to dry without wringing it. Two of my party — a lady 

 and gentleman — have worn garments thus treated in the wildest 

 storm of wind and rain without getting wet. The rain hangs upon 

 the cloth in globules. In short, they are really waterproof. The 

 gentleman, a fortnight ago, walked nine miles in a storm of rain and 

 wind such as you rarely see in the south, and when he slipped off 

 his over-coat, his under-clothes were as dry as when he put them on. 

 This is, I think, a secret worth knowing; for cloth, if it can be made 

 to keep out wet, is in every way better than what we know as water- 

 proofs. 



Prof. Phiu — I have seen a piece of common mosquito netting 

 with the meshes 1-1 6th of an inch wide laid over a tumbler, and a 

 teaspoonful of water poured upon it, and the water stood in a large 

 globule and did not fall through. The material had been rendered 

 repellant by some preparation, so that the water will not pass through 

 unless under pressure. But the air passes through freely. 



The President — The great objection to rubber waterproof is that 

 it is air-proof also. I remember that when I was in college, I bought 

 a pair of india-rubber boots with very long legs, to keep my feet dry 

 when walking in deep snow, but, to my surprise, found that my feet 

 were wet. I ascertained that it was because the perspiration could 

 not escape. It is unhealthy to wear india-rubber clothes, for they 

 retain the perspiration. But the prepared cloth allows the perspira- 

 tion to pass off freely. 



Dr. Yan der Weyde said that in wearing clothing which retained 

 the perspiration, we really poison ourselves ; because the perspiration 

 is a substance which nature is trying to expel, and which it is an 

 injury to retain in the system. 



IY. Wonderful Rapidity of Photographic Action. 

 The image of the full moon can be fixed in less than one-fourth of 

 a second, and that of the sun instantaneously. According to the 

 experiments of Mr. Waterhouse, a space of time no longer than one 

 twenty- seventh-thousandth of a second is required to fix the solar 

 image. Even this small fraction, however, inconceivably short as it 



