Spies of the Kaiser by William le Queux

Ray Raymond had been engaged watching the house of Hermann Hartmann in Pont Street ever since our discovery of the secret store of arms and ammunition down at Chiswick.

I had been absent at Devonport, keeping observation upon the movements of two Germans who had once or twice paid visits to Hartmann, and who had evidently received his instructions personally. The two men in question were known to us as spies, for with two other compatriots we had found them, only three months before, busily engaged in preparing a plan of the water-mains of East London, in order that, in case of invasion, some of the German colony could destroy the principal mains and thus deprive half the metropolis of drinking-water.